Updates the build script to point to the working jruby.

I was trying to follow the instructions to [manually set up a development environment](https://github.com/gocd/documentation/blob/master/3/3.1.md#312-setting-it-up-manually) on Mac OS X and the command `./bn clean` failed consistently with:

> Could not find i18n-0.6.1 in any of the sources
> Run `bundle install` to install missing gems.

It looks like `./tools/jruby-1.7.3` has some missing gems, so I found another jruby installation under `./tools/jruby` folder and it just worked.