Gem update can update to prereleases again

Previously this worked (I think) by updating to the next prerelease
automatically.  This broke with the recent changes for prereleases.

Now RubyGems checks for a prerelease version to update to (as
indicated by the requirement given to update_gems) and creates a new
installer with the option enabled.

This allows RubyGems to properly resolve the dependencies necessary to
update the gem.

The test for this change is poor as I can't see how to reproduce the
failure indicated in #1028 as it is complex.  Instead I just check to
see if the flag is correct as I trust the resolver.  A test duplicating
the failure in #1028 seems like it would result in a functional test
anyhow, so this is probably OK.

Fixes #1028