chore(composer) add PHP version requirement

Define minimal supported version of PHP needed into composer.json

Address hoaproject/Central#70.

Author: vonglasow

composer.json

@@ -27,6 +27,7 @@
         "source": "https://central.hoa-project.net/Resource/Library/Compiler"
     },
     "require": {
+        "php"            : ">=5.5.0",
         "hoa/consistency": "~1.0",
         "hoa/exception"  : "~1.0",
         "hoa/file"       : "~1.0",