chore(package): update mocha to version 6.2.1

Closes #56

Author: greenkeeper[bot]

package.json

@@ -30,7 +30,7 @@
   },
   "devDependencies": {
     "docco": "^0.8.0",
-    "mocha": "^5.0.0",
+    "mocha": "^6.2.1",
     "reify": "^0.18.1",
     "terser": "^3.16.0"
   },