Composer: Update PHP requirement to 7.1.3

composer.json

@@ -13,22 +13,29 @@
             "role": "Developer"
         }
     ],
+    "support": {
+        "issues": "https://github.com/getgrav/grav-plugin-admin/issues",
+        "irc": "https://chat.getgrav.org",
+        "forum": "http://getgrav.org/forum",
+        "docs": "https://github.com/getgrav/grav-plugin-admin/blob/master/README.md"
+    },
     "require": {
-        "php": ">=5.6.4",
+        "php": ">=7.1.3",
+        "ext-json": "*",
         "composer/semver": "^1.4",
         "fguillot/picofeed": "@stable"
     },
     "require-dev": {
-        "codeception/codeception": "^2.1",
-        "fzaninotto/faker": "^1.5",
-        "symfony/yaml": "~3.4",
-        "symfony/console": "~3.4",
-        "symfony/finder": "~3.4",
-        "symfony/event-dispatcher": "~3.4"
+        "codeception/codeception": "^2.4",
+        "fzaninotto/faker": "^1.8",
+        "symfony/yaml": "~4.1",
+        "symfony/console": "~4.1",
+        "symfony/finder": "~4.1",
+        "symfony/event-dispatcher": "~4.1"
     },
     "config": {
         "platform": {
-            "php": "5.6.4"
+            "php": "7.1.3"
         }
     },
     "scripts": {