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.
It's not directly related to NuGet, but without it any HTTPS connections from Mono will fail as no certificate root is trusted by default, this imports Mozillas list of trusted roots. You may not need it, but it's part of a proper Mono setup and many people are confused when secure connections don't work :smile: