chore(package): update mocha to version 6.1.2

Closes #17

Author: greenkeeper[bot]

package.json

@@ -34,6 +34,6 @@
   "devDependencies": {
     "eslint": "^4.1.0",
     "chai": "^4.0.1",
-    "mocha": "^3.2.0"
+    "mocha": "^6.1.2"
   }
 }