Update minimum PHP version

This version of the code has only be tested against PHP 5.6 or higher.

Author: brianlmoon

composer.json

@@ -15,7 +15,7 @@
         "issues": "https://github.com/brianlmoon/net_gearman/issues"
     },
     "require": {
-        "php": ">=5.2.0"
+        "php": ">=5.6.0"
     },
     "autoload": {
         "classmap": [ "Net/Gearman" ]