zeromq: Add an option to build with zeromq 2.2.x

This allows you to install/link the stable 2.x release by specifying `--zeromq-2`. I'm not sure of the option name, but I found this handy to help test code against both versions to maintain backward-compatibility.

With this change, you can install both simultaneously and switch between them like so:

```bash
# Install both versions:
brew install zeromq --zeromq-2 && brew unlink zeromq
brew install zeromq

# Switch to version 2.x:
brew unlink zeromq && brew link zeromq --zeromq-2

# Switch to version 3.x:
brew unlink zeromq && brew link zeromq
```