Unicorn worker killer not working

[apchester]

I have a Rails 3.2.19 application running on ruby 1.9.3 and have just installed unicorn killer to restart the worker processes.
I have added the line to the Gemfile and placed the following lines into config.ru as directed by the readme:

require 'unicorn/worker_killer'
use Unicorn::WorkerKiller::MaxRequests, 500, 1000, true

However restarting the application causes Unicorn to continually restart after failing on the require line. with the error below

config.ru:4:in require': cannot load such file -- unicorn/worker_killer (LoadError) from config.ru:4:inblock in

'

I think that this is the same issue as #35 although I am on a much older version of ruby/rails.

[apchester]

It appears that explicitly stopping and starting the unicorn process solves this issue.