Sidekiq "needs X11" in a rails project

Sidekiq "needs X11" in a rails project

I'm trying to use sidekiq in a project but have run into an unusual
problem that makes no sense. I've got multiple rails applications on my
laptop, many of which are using sidekiq, and are running fine without any
problems.
This application also uses sidekiq, and used to work however I'm not
running into a problem. I've not touched the project for a while, but the
only difference between now, and when it used to work is the upgrade from
OS X Lion, to Mountain Lion. This however hasn't effected other projects,
so I'm not sure what's going on. When I try to launch Sidekiq, I get an
alert stating
To open "sidekiq," you need to install X11.
Would you like to install X11 now?
It's the standard Apple alert, however I don't understand why Sidekiq is
supposedly trying to use X11, and why it isn't using it on my other
projects?
I'm using
Rails 3.2.8
Ruby 1.9.3p125
Sidekiq 2.13.1
Any help would be really appreciated :)