chore(package): update chai to version 4.1.2

Closes #10

Author: greenkeeper[bot]

package.json

@@ -8,7 +8,7 @@
     "request": "~2.78.0"
   },
   "devDependencies": {
-    "chai": "~3.5.0",
+    "chai": "~4.1.2",
     "istanbul": "~0.4.5",
     "mocha": "~3.2.0",
     "mocha-lcov-reporter": "~1.2.0",