Upgrade Rails to 3.2.18

This Rails releases fixes a security issue with globbed routes. For more
detail please see:

http://weblog.rubyonrails.org/2014/5/6/Rails_3_2_18_4_0_5_and_4_1_1_have_been_released/
Merge pull request #585 from alphagov/rails-3-2-18

Upgrade Rails to 3.2.18