Cleanup composer.json

- Use Illuminate packages instead of laravel/framework
- Require "php" >= 7.1
- Add "prefer-stable" so stable packages are used by default

composer.json

@@ -15,7 +15,10 @@
         }
     ],
     "require": {
-        "laravel/framework": "^6.0|^7.0|^8.0",
+        "php": "^7.1",
+        "illuminate/http": "^6.0|^7.0|^8.0",
+        "illuminate/routing": "^6.0|^7.0|^8.0",
+        "illuminate/support": "^6.0|^7.0|^8.0",
         "aws/aws-php-sns-message-validator": "^1.6"
     },
     "autoload": {
@@ -44,5 +47,6 @@
     "config": {
         "sort-packages": true
     },
-    "minimum-stability": "dev"
+    "minimum-stability": "dev",
+    "prefer-stable": true
 }