Upgrade to Rails 3.2.18

The first step of upgrading to Rails 4
Upgrade most dependencies, but lock shoulda-context for now

Upgrading it seems to break one test.
Open the flood gates and upgrade to the latest shoulda

Shoulda done this when we dropped Ruby 1.8 support.
Upgrade rubyzip

Another result of dropping Ruby 1.8 support.
Upgrade exception_notifier
Remove dead comment from Gemfile
Merge pull request #1497 from ferrous26/rails-upgrade

Rails upgrade to 3.2.18