Automate the process of updating the Roslyn version

It feels like we should be able to "insert" a new Roslyn version to this package daily, including updating any changelogs etc. Even if prerelease only actually goes out once a week or so, it feels like we would get data about any issues with integrating the new Roslyn versions more promptly and thoroughly this way.

With the FBP work going on, we also have a need to get new bug fixes and features out to dogfooders more promptly, who are willing to manually install VSIXes in order to try things out.