chore(package): update mocha to version 6.2.0

Closes #24

Author: greenkeeper[bot]

package.json

@@ -41,7 +41,7 @@
   "devDependencies": {
     "chai": "^3.4.0",
     "eslint": "^3.17.1",
-    "mocha": "^3.2.0"
+    "mocha": "^6.2.0"
   },
   "scripts": {
     "lint": "eslint test src",