Reduce requirement to PHP5.5

Author: minhd

composer.json

@@ -14,7 +14,7 @@
         }
     ],
     "require": {
-        "php": "~5.6|~7.0",
+        "php": "~5.5|~7.0",
         "guzzlehttp/guzzle": "^6.2",
         "symfony/console": "^3.1",
         "symfony/filesystem": "^3.1",