I think this warning comes from narray - at least I only see it with warnings enabled and requiring narray.
maasha@mel:~$ ruby -wr narray -e ''
lib/complex.rb is deprecated
maasha@mel:$ ruby --version
ruby 1.9.3p392 (2013-02-22 revision 39386) [x86_64-darwin11]
maasha@mel:$ gem --version
1.8.23
maasha@mel:~$ gem list --local | grep narray
narray (0.6.0.7)