[Fix #477] Add instructions for updating Prezto
[Fix #479] Start only one ssh-agent instance
Merge remote-tracking branch 'upstream/master'

* upstream/master:
  [Fix #479] Start only one ssh-agent instance
  [Fix #477] Add instructions for updating Prezto

Conflicts:
	README.md
	prezto/modules/completion/external
	prezto/modules/history-substring-search/external
	prezto/modules/syntax-highlighting/external