Update clean-css dependency

Author: jneurock

package.json

@@ -1,6 +1,6 @@
 {
   "name": "ember-cli-mirage-graphql",
-  "version": "0.2.1",
+  "version": "0.2.2",
   "description": "A library for mocking GraphQL with Ember CLI Mirage",
   "keywords": [
     "ember-addon"
@@ -32,6 +32,7 @@
     "@ember/optional-features": "^1.0.0",
     "babel-eslint": "^10.0.3",
     "broccoli-asset-rev": "^3.0.0",
+    "clean-css": "^4.1.11",
     "coveralls": "^3.0.2",
     "ember-apollo-client": "^2.0.0-beta.7",
     "ember-cli": "~3.13.1",