bumped minimum version to PHP 7.1

Author: fabpot

composer.json

@@ -16,13 +16,13 @@
         }
     ],
     "require": {
-        "php": ">=5.5.9",
+        "php": "^7.1.3",
         "symfony/polyfill-mbstring": "~1.0"
     },
     "require-dev": {
-        "symfony/config": "~2.8|~3.0|~4.0.0",
-        "symfony/intl": "^2.8.18|^3.2.5|~4.0.0",
-        "symfony/yaml": "~3.3|~4.0.0",
+        "symfony/config": "~2.8|~3.0|~4.0",
+        "symfony/intl": "^2.8.18|^3.2.5|~4.0",
+        "symfony/yaml": "~3.3|~4.0",
         "psr/log": "~1.0"
     },
     "conflict": {