[TASK] Update to PHPUnit 6 (#48)

Author: oliverklee

composer.json

@@ -26,7 +26,7 @@
     "psr/log": "1.0.0"
   },
   "require-dev": {
-    "phpunit/phpunit": "3.7.*",
+    "phpunit/phpunit": "^6.1",
     "squizlabs/php_codesniffer": "^2.6"
   },
   "autoload": {

core/tests/EmailUtilTest.php

@@ -2,8 +2,9 @@
 namespace phpList\test;
 
 use phpList\EmailUtil;
+use PHPUnit\Framework\TestCase;
 
-class EmailUtilTest extends \PHPUnit_Framework_TestCase
+class EmailUtilTest extends TestCase
 {
     public function setUp()
     {

core/tests/PassTest.php

@@ -1,14 +1,12 @@
 <?php
 namespace phpList\test;
 
-use phpList\Pass;
-
-// Symfony namespaces
+use PHPUnit\Framework\TestCase;
 use Symfony\Component\Config\FileLocator;
 use Symfony\Component\DependencyInjection\ContainerBuilder;
 use Symfony\Component\DependencyInjection\Loader\YamlFileLoader;
 
-class PassTest extends \PHPUnit_Framework_TestCase
+class PassTest extends TestCase
 {
     public function setUp()
     {