Upgrade Rails to tip of 3-2-stable branch

This includes the security fixes in 3.2.12.
However, 3.2.12 itself was just 3.2.11 plus CVE fixes, and
retained the habtm bugs we were hitting.

This advances us to the most recent commit on 3-2-stable, which
includes the security fixes but also has the habtm fix.
Merge pull request #404 from n1zyy/rails3.2.12-plus

Upgrade Rails to tip of 3-2-stable branch