Update PHP minimum

Author: schlessera

composer.json

@@ -9,7 +9,7 @@
     "license": "MIT",
     "type" : "phpcodesniffer-standard",
     "require": {
-        "php": ">=5.4",
+        "php": ">=5.6",
         "behat/behat": "^2.5.5",
         "dealerdirect/phpcodesniffer-composer-installer": "^0.4.3 || ^0.5 || ^0.6.2 || ^0.7",
         "php-parallel-lint/php-console-highlighter": "^0.5",