Upgrade rails to 3.2.16

All the tests are passing still, the only obvious change is that now
running the tests gives the following warning:

[deprecated] I18n.enforce_available_locales will default to true in the future. If you really want to skip validation of your locale you can set I18n.enforce_available_locales = false to a void this message.

It's not entirely clear which bit of code is causing this warning, but
it doesn't seem to be affecting anything, so I think it's ok to ignore
it for now.
Explicitly set I18n enforce_available_locales to false

This removes the deprecation warning and is the default behaviour in
i18n <= 0.6.8.

See http://stackoverflow.com/a/20381730/372120