chore(package): update chai to version 4.0.2 (#11)

Author: greenkeeper[bot]

package.json

@@ -26,7 +26,7 @@
   "license": "MIT",
   "readmeFilename": "README.md",
   "devDependencies": {
-    "chai": "^3.5.0",
+    "chai": "^4.0.2",
     "istanbul": "^0.4.5",
     "mocha": "^3.3.0"
   }