chore(package): update chai to version 4.1.2

Closes #10

Author: greenkeeper[bot]

package.json

@@ -22,7 +22,7 @@
   "license": "MIT",
   "devDependencies": {
     "babel-preset-es2015": "^6.3.13",
-    "chai": "^3.4.1",
+    "chai": "^4.1.2",
     "eslint": "^3.10.2",
     "gulp": "^3.9.0",
     "gulp-babel": "^6.1.1",