chore(package): update commitizen to version 4.0.0

Closes #17

Author: greenkeeper[bot]

package.json

@@ -33,7 +33,7 @@
   "devDependencies": {
     "better-than-before": "^1.0.0",
     "chai": "^4.0.0",
-    "commitizen": "2.9.6",
+    "commitizen": "4.0.0",
     "conventional-changelog-core": "^1.5.0",
     "coveralls": "^3.0.0",
     "ghooks": "1.3.2",