update php requirement and use newer PHPUNIT

Author: pfefferle

composer.json

@@ -18,10 +18,10 @@
   },
   "minimum-stability": "dev",
   "require": {
-    "php": ">=5.4",
+    "php": ">=5.6",
     "mf2/mf2": ">=0.2"
   },
   "require-dev": {
-    "phpunit/phpunit": "4.8.*"
+    "phpunit/phpunit": "^6"
   }
 }