Add Code Climate badge to README

Hello:

Full disclosure, I work at Code Climate :)

I pulled down vcr recently to help with testing out our new test reporter gem, and noticed you reference Code Climate in your README. Nice.

You have a very nice GPA going right now (3.8), so thought you might enjoy showing it off with our badge. I find that good OSS codebases tend to attract contributions.

Anyhow, thanks for using Code Climate and keep up the good work.