Do not rehash on rbenv initialization

We install rbenv-gem-rehash, which will automatically rehash when you
install new binaries, so I can't conceive of why this is necessary. I've been
using --no-rehash for months now without issue, and there's a noticeable speed
up in shell initialization.
Add libreadline-dev to linux install

Without libreadline-dev installed, IRB and the rails console won't
function properly.
Update README.md about Ubuntu 13.10 support
Add C compiler installation instructions for 10.9

Reformat C compiler instructions for various OS X versions so people can scan by
their version of OS X.