Remove direct dependency on import-js

[lencioni]

Instead of having a direct dependency on import-js in the Atom plugin, I think we should require import-js to be separately installed like we do for all other editor integrations. This will make things consistent, and work well with Galooshi/import-js#491

The VSCode plugin has some good integrations for auto installing import-js, which we might want to borrow from here.