chore(package): update mocha to version 6.2.1

Closes #11

Author: greenkeeper[bot]

package.json

@@ -37,7 +37,7 @@
   "devDependencies": {
     "chai": "^3.5.0",
     "coveralls": "^3.0.0",
-    "mocha": "^3.1.2",
+    "mocha": "^6.2.1",
     "nock": "^9.0.2",
     "nyc": "^10.0.0",
     "standard": "^10.0.0",