chore(package): update commitizen to version 3.1.1

Closes #22

Author: greenkeeper[bot]

package.json

@@ -57,7 +57,7 @@
     "babel-preset-es2015": "^6.16.0",
     "babel-preset-stage-2": "^6.16.0",
     "babel-register": "^6.16.3",
-    "commitizen": "^2.8.6",
+    "commitizen": "^3.1.1",
     "conventional-changelog-cli": "^1.2.0",
     "coveralls": "^2.11.14",
     "cz-conventional-changelog": "^1.2.0",