Bump cross-env from 5.2.0 to 6.0.3 (#226)

Bumps [cross-env](https://github.com/kentcdodds/cross-env) from 5.2.0 to 6.0.3.
- [Release notes](https://github.com/kentcdodds/cross-env/releases)
- [Changelog](https://github.com/kentcdodds/cross-env/blob/master/CHANGELOG.md)
- [Commits](kentcdodds/cross-env@v5.2.0...v6.0.3)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Author: dependabot-preview[bot]

package-lock.json

@@ -30,7 +30,7 @@
     "babel-preset-es2015": "^6.24.1",
     "babel-preset-react": "^6.24.1",
     "clean-webpack-plugin": "^2.0.1",
-    "cross-env": "^5.2.0",
+    "cross-env": "^6.0.3",
     "eslint": "^5.16.0",
     "eslint-config-airbnb-base": "^13.2.0",
     "eslint-plugin-import": "^2.18.2",