Update specs to run cleanly without deprecation warnings under RSpec 3.0.x

[petergoldstein]

1. Applied transpec to convert to RSpec 3.0.x syntax
2. Updated rspec version in gemspec
3. Fixed a few remaining post-transpec issues.

[arsduo]

This is fantastic! I didn't know about transpec before, but am definitely going to make use of it on other projects. Thanks!

[petergoldstein]

I just rebased this branch against master - it should be fully merged and run green.

When I rebased I ran into a minor issue in the multipart_request_spec - spec was failing because a hash was being used to mock env, while in real requests env actually behaves like a Mash. The super call was failing when it couldn't find the body method. I added a simple stub to deal with this.

And yes, transpec is awesome. :)

[arsduo]

I found that myself today and fixed it -- turns out somewhere along the line they moved from using hashes to a dedicated Faraday::Env class.

This won't show as merged, since I manually merged it in and the diff has changed, but all the code changes are in master now. (Like here). Thanks again! I really appreciate it.