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, but we can't wait till then. 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. This will also improve performance on the generated IL code.
We won't need this line - it appears to be related to nuget.