Put single quotes around rake task with bracketed args for Zsh compatibility

Zsh doesn't play nicely with bracketed Rake arguments.

Because it's widely-used in the Rails community, it seems like putting single quotes around the Rake argument will help people avoid a silly error without adding much complexity for other users.

For more details on this Zsh/Rake interaction, see:

http://robots.thoughtbot.com/post/18129303042/how-to-use-arguments-in-a-rake-task