After upgrading your OS X to Yosemite and Homebrew libraries you may encouter that your Ruby installations won't work anymore.

You need to reinstall your Ruby versions. The problem? This won't work right away:

$ rbenv install 2.1.3
rbenv: /Users/pulleasy/.rbenv/versions/2.1.3 already exists
continue with installation? (y/N) y
Downloading ruby-2.1.3.tar.gz...
Installing ruby-2.1.3...

BUILD FAILED (OS X 10.10 using ruby-build 20130628-366-gc2b2e15)

Inspect or clean up the working tree at /var/folders/df/w6ymx_bn7dggtv2cz6wtnp2c0000gn/T/ruby-build.20141022104547.86661
Results logged to /var/folders/df/w6ymx_bn7dggtv2cz6wtnp2c0000gn/T/ruby-build.20141022104547.86661.log

Last 10 log lines:
checking for stdint.h... yes
checking for unistd.h... yes
checking minix/config.h usability... no
checking minix/config.h presence... no
checking for minix/config.h... no
checking whether it is safe to define __EXTENSIONS__... yes
checking for cd using physical directory... cd -P
checking whether CFLAGS is valid... no
configure: error: something wrong with CFLAGS=" -O3 -Wno-error=shorten-64-to-32  "
make: *** No targets specified and no makefile found.  Stop.

To fix the issue, run the installer with the following flag:

CC=/usr/bin/gcc rbenv install 2.1.3

and you should be good to go.

If the above doesn't work for you, try this:

RUBY_CONFIGURE_OPTS="--without-gcc --disable-install-rdoc" rbenv install 2.1.3