chore(package): update chai to version 4.1.1

Closes #23

Author: greenkeeper[bot]

package.json

@@ -42,7 +42,7 @@
     "simple-bufferstream": "~1.0.0"
   },
   "devDependencies": {
-    "chai": "^3.5.0",
+    "chai": "^4.1.1",
     "express": "^4.14.0",
     "mocha": "^3.3.0",
     "sinon": "^2.1.0",