Now that Homebrew (as of this writing) downloads version 4.03 of OCaml, that ultimately breaks installation of Emily using this tap. Eventually, you'll encounter an error involving sedlex, namely that the version needed for Emily is incompatible with OCaml 4.03.0, and only works with a 4.02.x version. So... you need to manually download, compile, and install a bunch of things.
The first thing you need to do is get a hold of OCaml 4.02.3 here because you can't get earlier versions of things via Homebrew. Once downloaded, move into that directory and run the following:
./configure