chore(package): update chai to version 4.0.2 (#396)

package.json

@@ -39,7 +39,7 @@
     "showdown": "bin/showdown.js"
   },
   "devDependencies": {
-    "chai": "^3.5.0",
+    "chai": "^4.0.2",
     "grunt": "^1.0.1",
     "grunt-contrib-clean": "^1.0.0",
     "grunt-contrib-concat": "^1.0.1",