Only set nocompatible once as early as possible

* Setting nocompatible has side effects
* Setting it again (even if set) repeats side effects
* Settings like ruler will be reset if set again
Add HISTFILE to save zsh history

History file needs to be specified or otherwise the history would not be saved
over multiple terminal sessions.
[Source](http://zsh.sourceforge.net/FAQ/zshfaq03.html#321)
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