[WIP] use gulp-umd instead of browserify-shim

[jochenberger]

Bundles created using browserify-shim do not work in AMD enviroments. https://github.com/eduardolundgren/gulp-umd creates proper UMD wrappers and should be able solve problems like https://github.com/JedWatson/react-select/issues/270. I don't have much experience with browserify/gulp/node, but this is what I've come up with. However, when trying the changes on react-select, I end up with an empty dist file.

[jochenberger]

Maybe rollup-stream is a better alternative: https://github.com/gulpjs/gulp/blob/master/docs/recipes/rollup-with-rollup-stream.md