Bump min. version of PHPUnit (require-dev).

composer.json

@@ -19,7 +19,7 @@
         "php": ">=5.3.9"
     },
     "require-dev": {
-        "phpunit/phpunit": "~4.0"
+        "phpunit/phpunit": "~4.8"
     },
     "suggest": {
         "ext-phpiredis": "Allows faster serialization and deserialization of the Redis protocol",