Use Travis CI for building tests

Hello,
I just set up Travis CI configuration for this project, which will automatically builds Xcode tests whenever a new commit pushed to GitHub.

To make it work, please go to https://travis-ci.org/ and login, then enable Travis CI building for this project (under account settings). Then merge in this pull request.

You could see how CI building will looks like on my fork https://travis-ci.org/dlackty/LDProgressView