Changed definitions of overridden functions to match the original (#49)

Summary:
In #46 I override 4 functions
of the MySqlGrammar and added a return type to the definition.
Occurred that this is a breaking change as all users that extended from our class and
override the same functions - also should add the return type.

src/Query/Grammar.php

@@ -145,7 +145,7 @@ protected function wrapJsonFieldAndPath($column)
      * @param  string  $sql
      * @return string
      */
-    protected function wrapUnion($sql): string
+    protected function wrapUnion($sql)
     {
         return 'SELECT * FROM ('.$sql.')';
     }
@@ -156,7 +156,7 @@ protected function wrapUnion($sql): string
      * @param  Builder  $query
      * @return string
      */
-    protected function compileUnions(Builder $query): string
+    protected function compileUnions(Builder $query)
     {
         $sql = '';
 
@@ -173,7 +173,7 @@ protected function compileUnions(Builder $query): string
      * @param  Builder  $query
      * @return string
      */
-    public function compileSelect(Builder $query): string
+    public function compileSelect(Builder $query)
     {
         $sql = parent::compileSelect($query);
 
@@ -203,7 +203,7 @@ public function compileSelect(Builder $query): string
      * @param    $offset
      * @return string
      */
-    protected function compileOffset(Builder $query, $offset): string
+    protected function compileOffset(Builder $query, $offset)
     {
         return $this->compileOffsetWithLimit($offset, $query->limit);
     }