Update Slash theme

I made some modifications to my fork of the Slash theme to improve the
hierarchy of the DOM for SEO and other third-party integration (e.g.
Readability). This commit brings those updates to my Octopress blog by
updating my submodule:

  cd .themes/slash
  git pull origin my-master
  cd ../../
  bundle exec rake install['slash']

This theme update would have blown away my customizations to the
navigation, but I preserved them by selectively staging the changes
created by the final install step.