Use Ruby 2.0.0 for all Travis builds

* Old travis.yml used ruby 2.0.0 for one build, 1.9.3 (default) for rest
* New travis.yml more clearly shows ruby version being tested against
* Add travis build status badge to README
* View of new output: https://travis-ci.org/jessieay/sham_rack/builds/6092942
Merge pull request #5 from jessieay/use-same-ruby-version-in-all-travis-builds

Use Ruby 2.0.0 for all Travis builds