phpunit 5.1.* -> ^4.8 to support php 5.4 & 5.5

composer.json

@@ -12,7 +12,7 @@
         "yiisoft/yii2": "^2.0"
     },
     "require-dev": {
-        "phpunit/phpunit": "5.1.*",
+        "phpunit/phpunit": "^4.8",
         "fphammerle/helpers": "^1.2"
     },
     "autoload": {