Adds back zeromq v3.x. The API in 4.x is not backward-compatible.

Copied from Homebrew/homebrew@c356bf77400b0d1ca83431330bf0750741fc24b1 in the main repo.

Closes #327.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>
zeromq3: removes head references since it doesn't make sense

Closes #327.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>
zeromq3: mark as keg-only

Closes #327.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>
zeromq22: mark as keg-only

Closes #327.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>