Add .bundle to .gitignore

This pull request simply deleted the `.bundle` folder (which right now simply contains an install log), and adds the folder to `.gitignore` so that it doesn't get added back in on subsequent installs.

Beyond being a standard practice (the `.bundle` file is computer-specific), it will prevent leaking system information from contributors (and likely would cause merge conflicts on Pull Requests), e.g. [`RedCloth (4.2.9) from /Users/Charles_Personal/.rvm/gems/ruby-2.1.0@playbook/specifications/RedCloth-4.2.9.gemspec`](https://github.com/benbalter/playbook/blob/gh-pages/.bundle/install.log)