Define node environment constant in Webpack configuration

[aduth]

This pull request seeks to resolve a warning which is logged to the console when visiting the Gutenberg editor using the production build:

You are currently using minified code outside of NODE_ENV === 'production'. This means that you are running a slower development build of Redux. You can use loose-envify (https://github.com/zertosh/loose-envify) for browserify or DefinePlugin for webpack (http://stackoverflow.com/questions/30030031) to ensure you have the correct code for your production build.

Testing instructions:

1. Run npm run build
2. Visit the Gutenberg screen
3. Verify no warnings or errors are logged to the browser developer tools console

Caveats:

You may find different warnings are logged if you have a SCRIPT_DEBUG active. This is expected, and we should discourage mismatched environments (production built editor script, debug script constant).

[youknowriad]

LGTM 🚢