Switch from Travis to GitHub actions

[jeroenk]

• Testing with CVC4 is disabled, as any version of CVC4 that can be downloaded results in internal Boogie errors.
• Bugle does not build cleanly with the latest GCC and Clang; warnings are generated for some of the LLVM 6.0 headers Bugle is built against.
• Deployment is removed by this commit. Given the low volume of commits and the fact that there are problems with CVC4 and Bugle.

This is so that I'll be able to do some testing on the open pull requests. Note that a similar change will need to be made to Bugle.

[jeroenk]

Thanks! It mostly saves me from dealing with an extra platform (Travis), and it was a good way to learn a bit about GitHub actions.