debian: update ruby binary extension install path

From Christian Hofstaedler, Debian bug 739120:

When ruby-defaults > 1:2.0 will be uploaded to sid, the version of
Ruby provided by the 'ruby' and 'ruby-dev' packages will change to
version 2.0.  This version change includes a change to the path for
binary extensions.

This patch works for Ruby 1.9.1 as well as Ruby 2.0, so it's safe to
apply now.