chore(package): update mocha to version 4.0.0 (#18)

Author: greenkeeper[bot]

package.json

@@ -29,7 +29,7 @@
     "update-notifier": "^2.1.0"
   },
   "devDependencies": {
-    "mocha": "^3.2.0",
+    "mocha": "^4.0.0",
     "proxyquire": "^1.7.11",
     "sinon": "^4.0.0",
     "standard": "^10.0.1",