bumped minimum version to PHP 7.1

Author: fabpot

composer.json

@@ -16,12 +16,12 @@
         }
     ],
     "require": {
-        "php": ">=5.5.9",
-        "symfony/filesystem": "~2.8|~3.0|~4.0.0"
+        "php": "^7.1.3",
+        "symfony/filesystem": "~2.8|~3.0|~4.0"
     },
     "require-dev": {
-        "symfony/yaml": "~3.0|~4.0.0",
-        "symfony/dependency-injection": "~3.3|~4.0.0"
+        "symfony/yaml": "~3.0|~4.0",
+        "symfony/dependency-injection": "~3.3|~4.0"
     },
     "conflict": {
         "symfony/dependency-injection": "<3.3"