chore(package): update mocha to version 6.1.1

Closes #26

Author: greenkeeper[bot]

package.json

@@ -51,7 +51,7 @@
     "nan": "^2.6.2"
   },
   "devDependencies": {
-    "mocha": "~3.5.0",
+    "mocha": "~6.1.1",
     "chai": "~4.1.1",
     "grunt": "~1.0.1",
     "grunt-mocha-test": "~0.13.2",