Fixes # 1647 - erroneous message that a newer prerelease package is available. also ensures we only show this message if you are looking at the latest stable release of the package, since that is the scenario where it is most relevant.
Merge pull request #1654 from NuGet/tilovell/1647-prereleaseAvailableMessageFix

Fixes # 1647 - erroneous message that a newer prerelease package is avai...