bumped minimum version to PHP 7.1

Author: fabpot

composer.json

@@ -16,9 +16,8 @@
         }
     ],
     "require": {
-        "php": ">=5.5.9",
-        "symfony/polyfill-php56": "~1.0",
-        "symfony/options-resolver": "~2.8|~3.0|~4.0.0",
+        "php": "^7.1.3",
+        "symfony/options-resolver": "~2.8|~3.0|~4.0",
         "ext-ldap": "*"
     },
     "autoload": {