Use stable version of Slim 3

Author: dopesong

composer.json

@@ -17,7 +17,7 @@
         "psr/http-message": "^1.0"
     },
     "require-dev": {
-        "slim/slim": "3.x-dev"
+        "slim/slim": "^3.0"
     },
     "autoload": {
         "psr-4": {