chore(package): update mocha to version 6.2.0

Closes #11

Author: greenkeeper[bot]

package.json

@@ -35,7 +35,7 @@
     "chai": "^4.1.2",
     "codecov": "^3.0.4",
     "eslint": "^4.18.0",
-    "mocha": "^5.2.0",
+    "mocha": "^6.2.0",
     "nyc": "^12.0.2",
     "size-limit": "^1.0.1"
   },