Use composer platform config

composer.json

@@ -5,7 +5,7 @@
     "license": "MIT",
     "type": "project",
     "require": {
-        "php": ">=7.1.3",
+        "php": "^7.1.3",
         "fideloper/proxy": "~4.0",
         "laravel/framework": "5.6.*",
         "laravel/tinker": "~1.0"
@@ -16,7 +16,7 @@
         "fzaninotto/faker": "~1.4",
         "mockery/mockery": "~1.0",
         "phpunit/phpunit": "~7.0",
-        "symfony/thanks": "^1.0"
+        "symfony/thanks": "~1.0"
     },
     "autoload": {
         "classmap": [
@@ -51,10 +51,11 @@
         ]
     },
     "config": {
+        "platform": {
+            "php": "7.1.3"
+        },
         "preferred-install": "dist",
         "sort-packages": true,
         "optimize-autoloader": true
-    },
-    "minimum-stability": "dev",
-    "prefer-stable": true
+    }
 }