Travis CI - mono build status

This is really useful - it enables CI for mono on linux so we can check build status for mono.

It will restore nuget packages and build the solution using xbuild.

Current build is failing as it requires a fix to NBug solution as it tries to issue xcopy command under linux.

Let me know if you need help in setting up Travis

Check current status here https://travis-ci.org/fraga/gitextensions