Set up travis-ci to test on GNUstep/Linux

[ericwa]

Since we first set up builds on Travis-ci back in 2013-2014, they added support for building the same project on both Linux and macOS: https://docs.travis-ci.com/user/multi-os/

If we are going to continue supporting CO on GNUstep we should set up automated builds with it, otherwise it's likely we'll inadvertently break GNUstep support.

[qmathe]

Sounds like a good idea. I was planning to set up a new GNUstep environment with: https://github.com/ngrewe/gnustep-boxes

However using Travis seems like a better plan. If we can figure out a way to use LLVM/Clang packages, this would be perfect. We'll probably have to play with the compiler/linker flags since on Ubuntu/Debian, these packages still install the old GNU runtime according to the doc Niels wrote in the link I pasted.

[qmathe]

The example that showcases a multi OS build matrix looks exactly like what we want to have.