fix command to install homebrew

Summary:
When setting up my new mac (Happy New Year!), `make` from khan-dotfiles
gave me an error installing ruby. The result of the old `curl` was returning
HTML instead of shell code.  Looking at the Homebrew site, there is a
(presumably) new suggested install command:  see http://brew.sh/

There may be something here I don't understand, since I don't
understand where the existing command came from. git blame suggests
you touched it last, so I'm hoping you'll have some context.

Test Plan: With this patch, `make` could successfully install ruby.

Reviewers: marcos, alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D5789