Update Agnoster theme to use new powerline font codepoints

See https://github.com/Lokaltog/powerlin://github.com/Lokaltog/powerline and http://lokaltog.github.com/powerline/fontpatching.html for info about the changes in powerline and font patching.

With this change the theme will be compatible with the patched fonts in https://github.com/Lokaltog/powerline-fonts