bumped minimum version to PHP 7.1

Author: fabpot

composer.json

@@ -16,11 +16,11 @@
         }
     ],
     "require": {
-        "php": ">=5.5.9",
+        "php": "^7.1.3",
         "symfony/polyfill-mbstring": "~1.0"
     },
     "require-dev": {
-        "symfony/css-selector": "~2.8|~3.0|~4.0.0"
+        "symfony/css-selector": "~2.8|~3.0|~4.0"
     },
     "suggest": {
         "symfony/css-selector": ""