Use Travis CI language support for .NET

Travis CI [now supports](http://blog.travis-ci.com/2014-12-10-community-driven-language-support-comes-to-travis-ci/) C#, F#, VB projects. It uses Mono internally, just like you did previously in your .travis.yml. I also added `sudo: false`, so builds get routed to the new [container infrastructure](http://blog.travis-ci.com/2014-12-17-faster-builds-with-container-based-infrastructure/).

I'm one of the contributors that worked on this for Travis, feel free to ask me any questions :)