plt-racket 5.92

Just FYI: I'm creating this PR a *little* prematurely in order to use brew's testing-bot.

I have yet to figure out whether we should:
1. use their "Mac" build and just manually install the files to where we expect
2. pass `--enable-macprefix --prefix=#{prefix}` to `configure`
3. pass `--enable-xonx --prefix=#{prefix}` to `configure`