ci: revert mocha to old version for support node6

Author: Scrum

package.json

@@ -12,7 +12,7 @@
     "istanbul": "^0.4.5",
     "jscs": "^3.0.7",
     "jshint": "^2.10.3",
-    "mocha": "^7.0.0"
+    "mocha": "^6.2.2"
   },
   "scripts": {
     "test": "npm run lint && mocha -R dot && npm run cover",