Add sourcemap config to uglify plugin
Add sourcemap config to uglify plugin

No sourcemaps have been generated in `dist` since `0.14.3`
https://github.com/webpack/webpack/issues/2704#issuecomment-228860162

CookPete/react-player,CookPete/react-player,SkinyMonkey/react-player,SkinyMonkey/react-player