chore(package): update grunt-contrib-clean to version 2.0.0

Author: greenkeeper[bot]

package.json

@@ -29,7 +29,7 @@
     "grunt": "^1.0.1",
     "grunt-babel": "^6.0.0",
     "grunt-banner": "^0.6.0",
-    "grunt-contrib-clean": "^1.0.0",
+    "grunt-contrib-clean": "^2.0.0",
     "grunt-contrib-uglify": "^3.0.0",
     "grunt-release-it": "^1.0.1",
     "jsdom": "^11.6.2",