Bump minimum PHP version to 7.1

Author: mostertb

composer.json

@@ -5,7 +5,7 @@
     "homepage": "https://github.com/afrihost/BaseCommandBundle",
     "type": "symfony-bundle",
     "require": {
-        "php": "~7.0",
+        "php": "~7.1",
         "symfony/symfony": "~3.0|~4.0",
         "monolog/monolog": "~1.10"
     },