by bundler. Similar to r65613, but fixing this more carefully
because here is not just inside tests but inside rubygems code.
http://ci.rvm.jp/results/trunk_gcc5@silicon-docker/1448239
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@65629 b2dd03c8-39d4-4d8f-98ff-823fe69b080e