Add travis integration

Created by: gyscos

This configures travis to build and test termion on the stable, beta and nightly branches.

I updated the rainbow example to work on stable (removed the step_by feature), which admittedly isn't as pretty. Possible solutions are:

• Wait for step_by to be stable before integrating travis
• Add conditional to only compile rainbow example on nightly
• Use manual multiplications for now, and update back to step_by once it is stable