Add .travis.yml script for Travis CI

Also added execute permissions to build.sh so it can be run right after checkout.

This is how it looks: https://travis-ci.org/akoeplinger/omnisharp-roslyn/builds/44104421
@nosami It's [brand new](http://blog.travis-ci.com/2014-12-10-community-driven-language-support-comes-to-travis-ci/) :tada: