chore(package): update mocha to version 6.2.1

Closes #18

Author: greenkeeper[bot]

package.json

@@ -12,7 +12,7 @@
   },
   "devDependencies": {
     "markdown-it": "^8.0.1",
-    "mocha": "^5.0.3"
+    "mocha": "^6.2.1"
   },
   "keywords": [
     "markdown",