chore(package): update mocha to version 6.1.0

Closes #2

Author: greenkeeper[bot]

package.json

@@ -10,7 +10,7 @@
     "test-ci": "istanbul cover ./node_modules/mocha/bin/_mocha test/**/*.js --report lcovonly -- -R spec && cat ./coverage/lcov.info"
   },
   "devDependencies": {
-    "mocha": "^5.1.1",
+    "mocha": "^6.1.0",
     "chai": "^4.1.2",
     "chai-spies": "^1.0.0",
     "istanbul": "0.4.5"