update selenium_driver to 2.32.0, firefox to 21, and safari from 5/10.6 to 6/10.8

I'm getting ~stable results for {win7_chrome,win7_firefox_21,linux_firefox_21} as I did before this change. 

I've actually been using this as a local hack for a while too, and time to modernize/formalize this. 

Note: osx_firefox_21 has some test failures about handling alerts in the safari driver that we had previously. 

Also with this change, sign-in-test is stable on all browsers (as was frontend-qunit-test which doesn't stress out selenium window handling).