Apply `defcustom` fixes

The first form was already a `defcustom` form, but didn't have type and
group specified.  The second form was turned into a `defcustom` form
additionally to these changes.
Turn string into a choice

The spliced expression takes `inf-ruby-implementations` and turns them
into a list of choices compatible with `M-x customize`.
Turn `inf-ruby-implementations` into custom form
Merge pull request #54 from wasamasa/feature-defcustom

Apply `defcustom` fixes