chore(package): update mocha to version 6.2.2

Closes #44

Author: greenkeeper[bot]

package.json

@@ -35,7 +35,7 @@
     "grunt-exec": "^2.0.0",
     "grunt-gh-pages": "^2.0.0",
     "grunt-mocha-test": "^0.13.2",
-    "mocha": "^4.0.0",
+    "mocha": "^6.2.2",
     "phantomjs-prebuilt": "^2.1.12",
     "webpack": "^3.5.0"
   },