[Yt-dev] Fix for Mac readline

I noticed that on a default install (just running install_script.sh), yt issues warnings about not having access to readline: WARNING: Readline services not available on this platform. WARNING: The auto-indent feature requires the readline library Inside the yt env, I ran this: $ pip install -U ipython $ pip install readline and everything is good now. I'm guessing this just means some simple mods to install_script.sh, but this works for now. Best, Casey