Build releases with latest Mono

Stolen from https://github.com/travis-ci/travis-build/pull/294 which will probably need to bake a little longer and we can't wait forever. This will fix https://github.com/OpenRA/OpenRA/issues/6597 and  https://github.com/OpenRA/OpenRA/issues/6599 whose root may be a miscompile. Mono 2.10 is ancient and unsupported. We should stop using it to build releases.