mix.copy() does not work in watch mode
When using `mix.copy` the directive to clean assets in command `copy.js` file causes all copied files to be deleted, but not re-copied when the watch task recompiles.
Steps to replicate:
`mix.copy( 'file.jpg', 'new/location/file.jpg' )`
Run `npm run watch`
Make a change to any other file in the stack. `new/location/file.jpg` will be deleted.