chore(package): update mocha to version 6.2.2 (#47)

Closes #42

Co-authored-by: greenkeeper[bot] <23040076+greenkeeper[bot]@users.noreply.github.com>

Author: gucong3000

package.json

@@ -41,7 +41,7 @@
   "devDependencies": {
     "chai": "^4.1.2",
     "codecov": "^3.1.0",
-    "mocha": "^5.2.0",
+    "mocha": "^6.2.2",
     "nyc": "^15.0.0",
     "postcss": "^7.0.2",
     "postcss-safe-parser": "^4.0.1",