Remove Gem.refresh when using to bundler-1.4 later

bundler-1.4.rc1 or later removed Bundler.rubygems.refresh. When we used bundler-1.4.rc1 with master of rbenv-bundler, rbenv-bundler rehash is failed by calling Bundler.rubygems.refresh.

PS. I don't understand that Bundler removed this method. ref https://github.com/bundler/bundler/commit/90611d4a8625884f0099d103e4e6cd2e1c5438c0 .