Laravel 5.6 updates

Travis config update

Removed HHVM script as Laravel no longer support HHVM after releasing 5.3

vendor/symfony/http-kernel/Tests/CacheClearer/ChainCacheClearerTest.php

@@ -39,21 +39,6 @@ class ChainCacheClearerTest extends TestCase
         $chainClearer->clear(self::$cacheDir);
     }
 
-    /**
-     * @group legacy
-     */
-    public function testInjectClearerUsingAdd()
-    {
-        $clearer = $this->getMockClearer();
-        $clearer
-            ->expects($this->once())
-            ->method('clear');
-
-        $chainClearer = new ChainCacheClearer();
-        $chainClearer->add($clearer);
-        $chainClearer->clear(self::$cacheDir);
-    }
-
     protected function getMockClearer()
     {
         return $this->getMockBuilder('Symfony\Component\HttpKernel\CacheClearer\CacheClearerInterface')->getMock();