Scala 2.12 compatibility.

Apache Spark has embraced the GA support for 2.12 since the version 2.4.1, that was released nearly a year ago.
The Amazon EMR v6.0.0 characterised by the adoption of Hadoop 3.2.1 (sic!) and Scala 2.12 (although the default JDK is still Coretto 8) has been released a month ago, hence I presume it would be nice to have cross-builds for the Lighthouse.

This PR was cherry-picked from datamindedbe#27, where you can observe the matrix builds, as currently the Circle CI config supports only 2.11.

Changelog:
- enable cross-version builds for the projects;
- fix compile-time warning.

build.sbt

@@ -6,7 +6,7 @@ import sbt.Opts.resolver.sonatypeStaging
 lazy val buildSettings = Seq(
   organization := "be.dataminded",
   scalaVersion := scala211,
-  crossScalaVersions := Nil,
+  crossScalaVersions := supportedScalaVersions,
   // Ensure code quality
   scalafmtOnCompile := true,
   // Memory settings to be able to test with Spark
@@ -15,14 +15,14 @@ lazy val buildSettings = Seq(
   javaOptions ++= Seq(
     "-Xms768M",
     "-Xmx2048M",
-    "-XX:+CMSClassUnloadingEnabled",
+    "-XX:+UseStringDeduplication",
+    "-XX:+UseG1GC",
     "-Dspark.sql.shuffle.partitions=2",
     "-Dspark.shuffle.sort.bypassMergeThreshold=2",
     "-Dlighthouse.environment=test"
   ),
   scalacOptions ++= Seq(
     "-deprecation",
-    "-optimize",
     "-unchecked",
     "-Ydelambdafy:inline",
     "-Ypartial-unification",
@@ -61,8 +61,7 @@ lazy val lighthouse = (project in file("lighthouse-core"))
 lazy val `lighthouse-testing` = (project in file("lighthouse-testing"))
   .settings(
     buildSettings,
-    libraryDependencies ++= Seq(sparkSql, sparkHive, scalaTest, betterFiles),
-    crossScalaVersions := supportedScalaVersions
+    libraryDependencies ++= Seq(sparkSql, sparkHive, scalaTest, betterFiles)
   )
 
 lazy val `lighthouse-demo` = (project in file("lighthouse-demo"))

lighthouse-core/src/main/scala/be/dataminded/lighthouse/config/LighthouseConfigurationParser.scala

@@ -29,7 +29,7 @@ class LighthouseConfigurationParser extends OptionParser[LighthouseConfiguration
 
   opt[String]('e', "environment")
     .action((environment, config) => config.copy(environment = environment))
-    .withFallback(fallbackEnvironment)
+    .withFallback(() => fallbackEnvironment())
     .validate(item => if (item.nonEmpty) success else failure("The configured environment for Lighthouse is empty"))
     .required()
 

project/build.properties

@@ -1 +1 @@
-sbt.version=1.3.7
+sbt.version=1.3.8