Resolve #70 : Removed support for PHP 5.5

Author: khelle

composer.json

@@ -21,7 +21,7 @@
         }
     ],
     "require": {
-        "php": ">=5.5.9",
+        "php": ">=5.6.7",
         "kraken-php/event": "0.4.*",
         "kraken-php/loop": "0.4.*",
         "kraken-php/stream": "0.4.*",