theme agnoster: update characters to match latest powerline fonts

Clearing up some confusion in issue https://github.com/robbyrussell/oh-my-zsh/issues/1906. Powerline has changed the Unicode characters for font patching, see their change here: https://github.com/Lokaltog/powerline/commit/53fbfe15fead8cc7598bcb4ee9714a221ab7e446 which is described here: https://github.com/Lokaltog/powerline/issues/4. This pull request is to comply with those changes in agnoster.zsh-theme, which relies on a Powerline patched font. Users should patch their font with the latest version of Powerline or download a pre-patched font here https://github.com/Lokaltog/powerline-fonts.
![screen shot 2014-01-29 at 3 52 02 pm](https://f.cloud.github.com/assets/2744313/2035024/720fbe22-8934-11e3-8483-f421fbe119e2.png)