use libarchive by preference

[thommay]

Fixes: #16

@kamaradclimber it'd be great if you could give this is a try (you'll need to have libarchive installed)

[kamaradclimber]

Thanks for the quick patch @thommay .
It work perfectly. Two things:

• you need ffi-libarchive in your bundle otherwise it crashes (instead of doing the fallback to the default mechanism)
• berkshelf cache has been damaged by the failed extraction of cookbooks before your patch, it needs to be cleaned up (but the error message is quite clear so it is not hard at all).

Thanks again!

[thommay]

Thanks for the review. Fallback should be fixed now.

I think I am probably going to steer clear of doing anything with the berks cache for now; if loads of users struggle to fix it up then we can fix that up later.