Vagrant folder added to .gitignore

I'm working through [issue 212](https://github.com/twitter/summingbird/issues/212), and realized I had changed the gitignore file to use my Vagrant workflow.  I wasn't sure if y'all wanted such a trivial change as its own PR, a separate commit in a PR, or buried in another commit since this change is side effect.  The contributing lists "small simple changes" per commit, so I thought I'd just open a PR and ask just to be safe.