Relax minimum required version of Slim

Author: tuupola

composer.json

@@ -24,7 +24,7 @@
         "henrikbjorn/phpspec-code-coverage" : "^1.0",
         "coduo/phpspec-data-provider-extension": "^1.0",
         "akeneo/phpspec-skip-example-extension": "^1.0",
-        "slim/slim": "^3.5"
+        "slim/slim": "^3.0"
     },
     "suggest": {
         "zendframework/zend-diactoros": "Used with Diactoros Factories",