calabash-ios gem should be attached to CI that runs the rspec test on pull requests and pushes

## motivation

In the past we've had commits that break ruby version compatibility and otherwise put the gem into an unstable state.

I recommend that we use travis ci [1] to run the rspec tests against the versions of ruby that we support.  

If a push or a pull request causes the rspec tests to fail, maintainers will get an email about the failing build.  The downside is more emails.  The upside is that we will all know when a bad commit has been pushed.

I have also add a build-status shield to the README.md that will display the current status of the CI build.

### Why CI and Why Travis?

I am using Jenkins + CI on a couple of side projects to do merge builds of pull requests and mark the requests as safe/unsafe the merge.  The results have been spectacular - the near instant feedback has really helped boost the morale of the team and improved our confidence in the products.

I would use my machines Jenkins for calabash-ios, but it is resource starved, on an unstable network, and private.

I think Travis CI is a good solution.  There is nearly zero configuration overhead and the GitHub integration is seamless.  At the moment I am testing ruby 1.9.3, 2.0, and 2.1 - these builds happen in parallel.  The tests take ~5 minutes to run and the feedback time is ~6 minutes from push to build results.

## todo

- [ ] @krukow Karl, you need to review this.  
- [ ] @jmoody if approved, add calabash/calabash-ios to travis-ci (ie install webhook)

## links

- [1] https://travis-ci.org/
- [2] the travis-ci build for my fork jmoody/calabash-ios https://travis-ci.org/jmoody/calabash-ios
- [3] example of README.md badges https://github.com/gh-unit/gh-unit/blob/master/README.md