add php version constraint for 5.4.0

Author: igorw

composer.json

@@ -1,5 +1,6 @@
 {
     "require": {
+        "php": ">=5.4.0",
         "react/socket": "0.2.*",
         "monolog/monolog": "1.2.*"
     },
@@ -12,4 +13,4 @@
     "config": {
         "bin-dir": "bin/"
     }
-}
+}