Fix out of date README on rake, rspec commands

`rake` and `rspec` commands that wrapped `zeus` were removed in 00dbd15.
Show only basename and git branch in prompt

* Switch order from "[git branch][pathname]" to "basename git branch %".
* Remove noisy brackets.

Example old:

    [master][~/dev/thoughtbot/dotfiles]

Example new:

    dotfiles dc-prompt %