Update build.jl

Change homebrew package name from `zeromq` -> `zeromq32`, now that zeromq 4.0 is out.

Note that the `zeromq` package will still work, it will just constantly tell users that it's outdated, because Homebrew expects version 4.0 to be installed, not 3.2.4.  (I force it to install 3.2.4 if a package asks for just plain old `zeromq`, so things are always functional)