Upgrades Rails from 2.3.4 to 2.3.18

Upgrades Rails from [2.3.4](https://github.com/gocd/gocd/blob/master/server/webapp/WEB-INF/rails/vendor/rails/railties/lib/rails/version.rb) to 2.3.18.

The summary of changes is:
 - update rspec to use `tools/jruby-1.7.11` and removes the `tools/jruby`.
 - upgrade `jruby-complete` to match the same ruby version as `jruby-1.7.11`.
 - downgrade `jruby-1.7.11` rubygems version to be compatible with Rails 2.3.x.
 - introduces Bundler to vendorize gems.

After the changes the app runs on Rails 2.3.18 with Bundler and:
- **tests:** runs on jruby-1.7.11 (ruby-1.8.7p370 / rubygems 1.8.24)
- **production:** runs on jruby-complete-1.7.3 ruby-1.8 compat mode (ruby-1.8.7p370 / rubygems 1.8.24)

Next steps: gradually upgrade to Rails 3.x, then 4.x, then use `jruby` in 1.9 mode.

Investigation related to #130 and #149.