Add support for Travis-CI

Add support for building via Travis-CI (https://travis-ci.org/).

All that is needed is the .travis.yml config file and then flipping on the bit in the repository settings. Travis can then automatically check that the linux builds compile for pushes, pull requests, or both.

I intentionally only added building for a representative subset of the sdks, one from each major engine version (for whatever meaning that really has) to keep build times down while still getting good coverage.