Bump minimum PHP to 5.5.9

Author: GrahamCampbell

composer.json

@@ -16,7 +16,7 @@
         }
     ],
     "require": {
-        "php": "^5.4 || ^7.0",
+        "php": "^5.5.9 || ^7.0",
         "phpoption/phpoption": "^1.5",
         "symfony/polyfill-ctype": "^1.9"
     },