Fix dependency constraint because of a BC break in Slim 3.2.0

Author: mnapoli

composer.json

@@ -17,7 +17,7 @@
         "php": "~5.6|~7.0",
         "php-di/php-di": "^5.2.0",
         "php-di/invoker": "^1.2.0",
-        "slim/slim": "^3.2.0"
+        "slim/slim": "^3.2.1"
     },
     "require-dev": {
         "phpunit/phpunit": "~5.1"