diff --git a/.travis.yml b/.travis.yml
index 8d42a57..e81546b 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -3,15 +3,24 @@ language: php
 matrix:
   fast_finish: true
   include:
-    - php: 5.5
     - php: 5.6
       env:
-        - CODE_COVERAGE=true
+        - COMPOSER_FLAGS="--prefer-lowest --prefer-dist --no-interaction"
+    - php: 5.6
+    - php: 7.0
+      env:
+        - COMPOSER_FLAGS="--prefer-lowest --prefer-dist --no-interaction"
     - php: 7.0
+    - php: 7.1
+      env:
+        - COMPOSER_FLAGS="--prefer-lowest --prefer-dist --no-interaction"
+    - php: 7.1
+      env:
+        - CODE_COVERAGE=true
 
 install:
    - composer self-update
-   - composer update
+   - composer update $COMPOSER_FLAGS
    - composer info
 
 script:
diff --git a/composer.json b/composer.json
index ed8e002..239e7fe 100644
--- a/composer.json
+++ b/composer.json
@@ -10,10 +10,10 @@
         }
     ],
     "require": {
-        "php": "~5.5 || ~7.0",
+        "php": "^5.6 || ^7.0",
         "doctrine/common": "^2.6",
         "ramsey/uuid": "^3.1",
-        "symfony/event-dispatcher": "^2.6 || ^3.0",
+        "symfony/event-dispatcher": "^2.6 || ^3.4 || ^4.0",
         "mtdowling/cron-expression": "^1.1"
     },
     "require-dev": {
diff --git a/src/Task/Runner/TaskRunner.php b/src/Task/Runner/TaskRunner.php
index 8b08817..1d93d30 100644
--- a/src/Task/Runner/TaskRunner.php
+++ b/src/Task/Runner/TaskRunner.php
@@ -208,7 +208,7 @@ private function finalize(TaskExecutionInterface $execution, $start)
         // invalid (clear in doctrine) after handling an execution.
         $execution = $this->taskExecutionRepository->findByUuid($execution->getUuid());
 
-        if ($execution->getStatus() !== TaskStatus::PLANNED) {
+        if (TaskStatus::PLANNED !== $execution->getStatus()) {
             $execution->setEndTime(new \DateTime());
             $execution->setDuration(microtime(true) - $start);
         }
diff --git a/src/Task/Scheduler/TaskScheduler.php b/src/Task/Scheduler/TaskScheduler.php
index e0142b6..c18c3f1 100644
--- a/src/Task/Scheduler/TaskScheduler.php
+++ b/src/Task/Scheduler/TaskScheduler.php
@@ -107,7 +107,7 @@ protected function scheduleTask(TaskInterface $task)
             return;
         }
 
-        if ($task->getInterval() === null && 0 < count($this->taskExecutionRepository->findByTask($task))) {
+        if (null === $task->getInterval() && 0 < count($this->taskExecutionRepository->findByTask($task))) {
             return;
         }
 
diff --git a/src/Task/Storage/ArrayStorage/ArrayTaskExecutionRepository.php b/src/Task/Storage/ArrayStorage/ArrayTaskExecutionRepository.php
index 6cc990b..ce271e5 100644
--- a/src/Task/Storage/ArrayStorage/ArrayTaskExecutionRepository.php
+++ b/src/Task/Storage/ArrayStorage/ArrayTaskExecutionRepository.php
@@ -89,7 +89,7 @@ function (TaskExecutionInterface $execution) use ($task) {
             }
         );
 
-        if ($filtered->count() === 0) {
+        if (0 === $filtered->count()) {
             return;
         }
 
@@ -107,7 +107,7 @@ function (TaskExecutionInterface $execution) use ($uuid) {
             }
         );
 
-        if ($filtered->count() === 0) {
+        if (0 === $filtered->count()) {
             return;
         }
 
@@ -153,7 +153,7 @@ public function findNextScheduled(\DateTime $dateTime = null, array $skippedExec
 
         $result = $this->taskExecutionCollection->filter(
             function (TaskExecutionInterface $execution) use ($dateTime, $skippedExecutions) {
-                return $execution->getStatus() === TaskStatus::PLANNED
+                return TaskStatus::PLANNED === $execution->getStatus()
                     && $execution->getScheduleTime() < $dateTime
                     && !in_array($execution->getUuid(), $skippedExecutions);
             }
diff --git a/src/Task/Storage/ArrayStorage/ArrayTaskRepository.php b/src/Task/Storage/ArrayStorage/ArrayTaskRepository.php
index 676f0b0..064ea31 100644
--- a/src/Task/Storage/ArrayStorage/ArrayTaskRepository.php
+++ b/src/Task/Storage/ArrayStorage/ArrayTaskRepository.php
@@ -106,7 +106,7 @@ public function findEndBeforeNow()
         return array_values(
             $this->taskCollection->filter(
                 function (TaskInterface $task) use ($now) {
-                    return $task->getLastExecution() === null || $task->getLastExecution() > $now;
+                    return null === $task->getLastExecution() || $task->getLastExecution() > $now;
                 }
             )->toArray()
         );
diff --git a/src/Task/TaskStatus.php b/src/Task/TaskStatus.php
index 00bab7d..cd2f165 100644
--- a/src/Task/TaskStatus.php
+++ b/src/Task/TaskStatus.php
@@ -17,9 +17,13 @@
 final class TaskStatus
 {
     const PLANNED = 'planned';
+
     const RUNNING = 'running';
+
     const COMPLETED = 'completed';
+
     const ABORTED = 'aborted';
+
     const FAILED = 'failed';
 
     /**
diff --git a/tests/Unit/Runner/PendingExecutionFinderTest.php b/tests/Unit/Runner/PendingExecutionFinderTest.php
index 6f9f846..d2aae7f 100644
--- a/tests/Unit/Runner/PendingExecutionFinderTest.php
+++ b/tests/Unit/Runner/PendingExecutionFinderTest.php
@@ -24,6 +24,7 @@
 class PendingExecutionFinderTest extends \PHPUnit_Framework_TestCase
 {
     const HANDLER = 'AppBundle\\Handler\\TestHandler';
+
     const LOCKING_HANDLER = 'AppBundle\\Handler\\LockingTestHandler';
 
     /**