From the npm docs, using a git URL:
git://github.com/<user>/<project>.git#<branch>
git://github.com/<user>/<project>.git#feature\/<branch>
As of NPM version 1.1.65, you can use a shorten github URL:
<user>/<project>#<branch>
UPDATE 2022
Don’t use git://
protocol for GitHub, as it is not supported
npm ERR! The unauthenticated git protocol on port 9418 is no longer supported.
npm ERR! Please see https://github.blog/2021-09-01-improving-git-protocol-security-github/ for more information.