chore(package): update mocha to version 6.2.0

Closes #9

Author: greenkeeper[bot]

package.json

@@ -37,7 +37,7 @@
     "node": ">=6"
   },
   "devDependencies": {
-    "mocha": "^3.2.0",
+    "mocha": "^6.2.0",
     "nock": "github:ierceg/nock#promisify-nock-back",
     "readme-badges": "^0.3.3",
     "readme-includes": "^0.2.9",