Allow installation on PHP 7.2

Author: davidwindell

composer.json

@@ -8,7 +8,7 @@
         }
     },
     "require": {
-        "php": "~5.5.0|~5.6.0|~7.0.0|~7.1.0"
+        "php": "~5.5.0|~5.6.0|~7.0.0|~7.1.0|~7.2.0"
     },
     "autoload": {
         "psr-4": {