Drop flatten-packages dependency

[jacobq]

Removing this will help eliminate the annoying warning:

npm WARN deprecated wrench@1.5.9: wrench.js is deprecated! You should check out fs-extra (https://github.com/jprichardson/node-fs-extra) for any operations you were using wrench for. Thanks for all the usage over the years.

I think that new npm flattens automatically, so it should be OK to get rid of this. This will probably also break node 4.x since that doesn't support current npm, so it'd be a breaking change.

[felixrieseberg]

I'm into this idea though, I think we should go forth and drop it.

[felixrieseberg]

And done!