Use new session cookie name, so upgrades to Rails >=3.1 don't error. Closes #456.

If you send a Rails <3.1 cookie to a Rails >=3.1 app, an
ArgumentError (dump format error) is raised. We can work around this
by changing our session cookie name, which stops the app from trying
to read the old cookie, and forces a new cookie to be created.
Merge remote-tracking branch 'origin/v1.x'
Merge remote-tracking branch 'origin/master' into coop