I have installed a new rails project like so:
$ rails new site
and it executes and reaches:
bundle install
<
This appears to be an issue relating to the system Ruby that is packaged with Mavericks (2.0.0p247), see details about the bug here: https://bugs.ruby-lang.org/issues/9624
I got around this by installing rbenv (or RVM if you prefer) and then installing Ruby 2.1.1 so as not to tamper with the system Ruby version. You will still get a warning during the installation of the atomic gem but it won't fail at least.
rbenv here: https://github.com/sstephenson/rbenv