-bash: __git_ps1: command not found

If you've not installed a newer version of Git on your Mac, start by doing that. You can grab the download from here: http://git-scm.com/download

Once it's installed, you should source git-completion.sh and git-prompt.sh from your ~/.profile (or ~/.bashrc, if you have it):

source /usr/local/git/contrib/completion/git-completion.bash
source /usr/local/git/contrib/completion/git-prompt.sh

That should fix the __git_ps1 error.

I'm not a Rubyist, but the rbenv error is coming from the fact that the setup in the dotfiles you are following is trying to set the default Ruby version to "1.9.3-p194". The rbenv tool is there to make a number of different Ruby versions available, but you have to install them first. In your case, I think you need to run this:

rbenv install 1.9.3-p194

After that, the version will be available, and you shouldn't see the warning anymore.