chore(package): update chai to version 4.0.0

Author: greenkeeper[bot]

package.json

@@ -39,7 +39,7 @@
     "toggl-api": "^1.0.1"
   },
   "devDependencies": {
-    "chai": "^3.5.0",
+    "chai": "^4.0.0",
     "codeclimate-test-reporter": "^0.4.0",
     "coffee-coverage": "^2.0.0",
     "coffee-script": "^1.11.1",