Use latest PHPUnit dev dependency.

composer.json

@@ -16,7 +16,7 @@
         "php": ">=5.2.4"
     },
     "require-dev": {
-        "phpunit/phpunit": "~3.7|~4.0",
+        "phpunit/phpunit": "~3.7|~4.0|~5.0",
         "fabpot/php-cs-fixer": "~1.6"
     },
     "autoload": {