This is what's on the page to get the the latest version:
brew install --HEAD neovim
Not sure if it's actually nighlty, but it seems like it's at least close.
NOTE: Not sure if this is the right way to do it.
NOTE: This will probably have problems with LUA stuff since those dependencies don't seem to be met.
Doubleclick on the `.tar.gz` file in your ~/Downloads folder which will extract/untar the archive into a folder named `nvim-osx64`
Then you can copy:
cp ~/Downlaods/nvim-osx64/bin/nvim /use/local/bin/nvim
Note that there are some other libraries the come down with the archive. No idea if you need to do something with them.