Force PHPUnit 4.0 for testing

Author: akrabat

composer.json

@@ -17,7 +17,8 @@
         "psr/http-message": "^1.0"
     },
     "require-dev": {
-        "slim/slim": "^3.0"
+        "slim/slim": "^3.0",
+        "phpunit/phpunit": "^4.0"
     },
     "autoload": {
         "psr-4": {