Update cross-env to the latest version 🚀 (#20)

* chore(package): update cross-env to version 4.0.0

https://greenkeeper.io/

* update yarn.lock

Author: greenkeeper[bot]

package.json

@@ -43,7 +43,7 @@
     "babel-plugin-transform-object-rest-spread": "^6.20.2",
     "babel-preset-es2015": "^6.18.0",
     "babel-preset-react": "^6.16.0",
-    "cross-env": "^3.2.3",
+    "cross-env": "^4.0.0",
     "eslint": "^3.15.0",
     "eslint-config-airbnb": "^14.1.0",
     "eslint-formatter-pretty": "1.1.0",