chore(package): update mocha to version 6.2.2

Closes #12

Author: greenkeeper[bot]

package.json

@@ -39,7 +39,7 @@
     "@types/node": "^10.1.2",
     "chai": "^4.1.2",
     "coveralls": "^3.0.1",
-    "mocha": "^5.2.0",
+    "mocha": "^6.2.2",
     "moment-timezone": "^0.5.17",
     "nyc": "^12.0.2",
     "prettier": "^1.13.0",