chore(package): update chai to version 4.1.1

Closes #52

Author: greenkeeper[bot]

package.json

@@ -36,7 +36,7 @@
   "devDependencies": {
     "bluebird": "^3.4.7",
     "browserify": "^14.1.0",
-    "chai": "^3.5.0",
+    "chai": "^4.1.1",
     "chai-as-promised": "^6.0.0",
     "es3ify": "^0.2.2",
     "istanbul": "^0.4.5",