Set up automatic testing with travis-ci

[donv]

This is to ensure that gosu-android works with different Android versions, at least api levels 10,15, and 16. It also protects against regressions for the tested features.

Ruboto is already being tested on travis-ci, so you can copy the setup from there.

[Era-Dorta]

I took a look into this and I got the basic ideas, a rake test, install android sdk, install the apps and check the return values. However I did not manage to get it all working for gosu_android, I got confuse with so many files a configurations in ruboto. So whenever I have more time I'll try it again.

[donv]

I would be happy to help. You have set up the automatic testing, so I think pull requests will be tested. I will try to make a proposal.

[donv]

I am a bit confused about what branch to use. "master" is not the most recently updated branch. Which branch should I use? I would prefer to use master.

[Era-Dorta]

Master has the same version the gem does. I use trunk for commits that are stable. Then I use other branches for development and if the changes work, I merge them back to trunk. I thought that was the standard way to do things with git. So yes, I think you can use master. Thank you for helping me out :-)