Update the "upgrading" guide for people coming from 3.0 and 3.1.

The latest versions for 3.1 and 3.2 are now 3.2.12 and 3.1.11 respectively, whereas the "Upgrading Ruby on Rails" guide still claimed the latest versions were 3.2.2 and 3.1.3.

Since several security related releases have happened since, it seemed prudent to point people looking to upgrade to the latest version.

Note that the Release Notes for 3.1 also claim the latest 3.1 version is 3.1.3. I did not change that, I wasn't sure if it's customary to keep those up to date. It also seemed less likely to cause problems.