I have found several threads where other users have had similar issues but none of them are seem to be like this.
After issuing the command:
rbenv in
The Homebrew package for gcc on Yosemite seems to be broken. That's why it doesn't work.
Had the same issue. Running:
CC=/usr/bin/gcc rbenv install 2.1.3
worked for me.
This has been fixed in ruby-build for rbenv. Please update by the following:
cd ~/.rbenv/plugins/ruby-build/ git pull