We originally chose to apply very strict versioning on the `rails` entry
in the Gemfile, because our future versioning policy was not strongly
defined.

Now it is, and our policy is very much designed on the expectation that
people will regularly update to the latest patch level in their release
series... so we should encourage that.

Of course, Gemfile.lock will do its job and prevent unplanned updates,
just as it does for every other gem in the bundle... but if you run
`bundle update`, we want to get you the latest bug/security fixes
without requiring a manual edit of the Gemfile entry.

Our current version could be a few different shapes, so it takes a bit
of work to find the right specifier, but in principle, we match anything
of the form x.y.*, where x.y matches our current release series.