Update PHPUnit to 6.0.

Author: jhedstrom

composer.json

@@ -22,7 +22,7 @@
         "composer/composer": "~1.0",
         "jakub-onderka/php-parallel-lint": "~0.8",
         "mikey179/vfsStream": "~1.2",
-        "phpunit/phpunit": "~5.6",
+        "phpunit/phpunit": "~6.0",
         "squizlabs/php_codesniffer": "~2.0"
     },
     "autoload": {

tests/DrupalInfoTest.php

@@ -16,11 +16,12 @@
 use Composer\Repository\WritableRepositoryInterface;
 use Composer\Script\Event;
 use DrupalComposer\Composer\DrupalInfo;
+use PHPUnit\Framework\TestCase;
 
 /**
  * @coversDefaultClass \DrupalComposer\Composer\DrupalInfo
  */
-class DrupalInfoTest extends \PHPUnit_Framework_TestCase
+class DrupalInfoTest extends TestCase
 {
     use InfoFileTrait;
 

tests/Writer/FactoryTest.php

@@ -6,11 +6,12 @@
 use DrupalComposer\Composer\Writer\Drupal;
 use DrupalComposer\Composer\Writer\Drupal7;
 use DrupalComposer\Composer\Writer\Factory;
+use PHPUnit\Framework\TestCase;
 
 /**
  * @coversDefaultClass \DrupalComposer\Composer\Writer\Factory
  */
-class FactoryTest extends \PHPUnit_Framework_TestCase
+class FactoryTest extends TestCase
 {
     use InfoFileTrait;