[2005] Update all the Rails!

For #2005

This fixes a problem encountered by anyone who had generated their
`spec/rails` test environment (occurs on first run) after #1970 was
applied. The problem was that I had updated only a few places where we
refer to the current Rails version to be tested. This commit changes
all of them, as well as bumps 3.2 and 3.1 to reflect the new release.

If you ran into the #2005 problem, please run these steps:
```sh
rm Gemfile.lock .rails-version
bundle
rm -rf spec/rails/
rspec spec
```