Support svn_client_export4().
Support svn_client_export4().

* subversion/bindings/swig/ruby/svn/client.rb
  (Svn::Client::Context#export): Use Svn::Client.export4 instead of
  Svn::Client.export3.

jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion