company-mode completion back-end for inf-ruby buffers.
NOTE: Now that inf-ruby 2.4.0 supports completion-at-point, this backend is deprecated.
You don't need to install anything extra, company-capf will work with inf-ruby.
Install this from Marmalade.
Then, in addition to the usual company-mode setup, add this to your init file:
(eval-after-load 'company
'(add-to-list 'company-backends 'company-inf-ruby))