Allow Swig to use Ruby 2.0 and 2.1.

Patch by: James McCoy <jamessan{_AT_}debian.org>

* build/ac-macros/swig.m4
  (SVN_FIND_SWIG): Adjust include path for Ruby 2.

* configure.ac: Allow Ruby 2.0 and 2.1.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1596882 13f79535-47bb-0310-9956-ffa450edef68