Berks info 1345

This is a proposed fix for #1345, which lets 'berks info' and 'berks upload' succeed when running with a vendored BERKSHELF_PATH.

There are a couple of comments added where I wanted to remove code that didn't seem to be doing anything. Since I'm a new to ruby, I would appreciate some feedback on those.

There is a change to setting Berkshelf's @berkshelf_path, because on OS X, the environment variable was never listened to. I'm not confident on this one, but without this change, I couldn't ever 'berks install' into ENV['BERKSHELF_PATH'].

There's a change to an rspec test that fails on mac because the value of the scratch dir is os-dependent. There's possibly a nicer way to mark up tests to not run on darwin? But, instead, I if-deffed on detection of darwin.

Cukes and rspec tests look ok, but it's possible we should have better coverage, especially around BERKSHELF_PATH which wasn't working for me on OS X?

Anyways, please take a look and let me know what you think.