chore(package): update chai to version 4.0.0 (#31)

Author: greenkeeper[bot]

package.json

@@ -49,7 +49,7 @@
     "babel-preset-es2015": "^6.6.0",
     "babel-preset-react": "^6.5.0",
     "babel-preset-stage-3": "^6.22.0",
-    "chai": "^3.5.0",
+    "chai": "^4.0.0",
     "enzyme": "^2.2.0",
     "flow-bin": "^0.46.0",
     "flow-copy-source": "^1.1.0",