Fixed broken README badge

@davideast - The README badge showed `?.?.?` instead of `0.3.0` because it kept trying to read from the `master` branch by default, but we use the `gh-pages` branch instead. I'd really like to rename the main branch as `master` to be consistent with our other repos. We would have to park [the demo](http://firebase.github.io/backfire/examples/todos/) on Firebase Hosting and update a few things in the README. Let's do this together next week.