chore(package): update mocha to version 6.2.1

Closes #54

Author: greenkeeper[bot]

package.json

@@ -29,7 +29,7 @@
     "chai": "^4.1.2",
     "dirty-chai": "^2.0.1",
     "eslint": "^4.11.0",
-    "mocha": "^4.0.1",
+    "mocha": "^6.2.1",
     "publish-please": "^2.2.0"
   }
 }