show progress of rdoc parsing to prevent timeout on chkbuild

http://c64b.rubyci.org/~chkbuild/ruby-trunk/log/20140520T030303Z.fail.html.gz

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46027 b2dd03c8-39d4-4d8f-98ff-823fe69b080e