[Fix #477] Add instructions for updating Prezto
[Fix #479] Start only one ssh-agent instance
Merge branch 'master' of https://github.com/sorin-ionescu/prezto into HEAD

* 'master' of https://github.com/sorin-ionescu/prezto:
  [Fix #479] Start only one ssh-agent instance
  [Fix #477] Add instructions for updating Prezto