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

Copied from Homebrew/homebrew@c356bf77400b0d1ca83431330bf0750741fc24b1 in the main repo.

Both the main `zeromq` package and this one have overlapping symlink paths (and the zeromq22 as well). Would it make sense to disable the linking by default on versioned formulas ?