chore(package): update chai to version 4.0.2

Closes #28

Author: greenkeeper[bot]

package.json

@@ -14,7 +14,7 @@
     "mocha": "~3.3.0",
     "sinon": "~1.17.6",
     "sinon-chai": "~2.8.0",
-    "chai": "~3.5.0",
+    "chai": "~4.0.2",
     "request": "~2.80.0"
   },
   "scripts": {