Adding @alext's rack_strip_client_ip gem to stop smart-answers blowing up

Explanation here - https://github.com/alphagov/whitehall/pull/810. We're seeing several instances of IpSpoofAttackError in Errbit.
Merge branch 'master' into fix_ip_spoofing_errors
Merge pull request #823 from alphagov/fix_ip_spoofing_errors

Adding rack_strip_client_ip gem to stop smartanswers blowing up