The current bundler version in Portage is 1.7.13, which was released more than 1 month ago (2015-02-07). There were 7 releases (not including beta and rc) since that date, and some tools like https://rails-assets.org/ already require more recent bundler version. Bundler 1.9 was released at 2015-03-20. Please update the version in portage if possible.
bundler-1.8.7 is now in the tree. We'll add bundler 1.9 versions later.
bundler 1.9.9 is now available.