Christmas 2012 gem clearout

Each gem we require via the Gemfile adds to the startup time of our app. We had a few lingering in the publisher Gemfile that we're not using at all, and a number of others that are rarely used so are easy to require explicitly.

Some people are beginning to advocate removing [the Bundler.require from application.rb](https://github.com/alphagov/publisher/blob/master/config/application.rb#L13) but I've decided not to go that far as it would require quite extensive testing.

I've not changed unicorn to be explicitly required but it might be worth looking at that before merging this.