fix composer requirement

Author: dhoffend

composer.json

@@ -18,7 +18,7 @@
         "psr/http-message": "^1.0"
     },
     "require-dev": {
-        "slim/slim": "dev-develop"
+        "slim/slim": "3.x-dev"
     },
     "autoload": {
         "psr-4": {