chore(deps): update dependency mocha to v6 (#37)

Author: renovate[bot]

package.json

@@ -31,7 +31,7 @@
   ],
   "devDependencies": {
     "markdown-toc": "^1.0.2",
-    "mocha": "^5.2.0"
+    "mocha": "^6.0.0"
   },
   "dependencies": {
     "@textlint-rule/textlint-rule-no-invalid-control-character": "^1.1.0",