I have installed a new rails project like so:
$ rails new site
and it executes and reaches:
bundle install
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I had this same problem and what worked for me was installing RVM, installing Ruby 2.1.1 (an upgrade from the 2.0 that comes packaged in Mavericks)...
rvm get stable
rvm reinstall 2.1.1
(use whatever the latest version is, for me that's 2.1.1)
...and then run rvm use 2.1.1 to make sure that I had switched to using that version.
After that, my gem installs ran smoothly.