Test on Racket 5.2, too.

Note: Someone pointed out that multiple `env` items in `.travis.yml`
should result in a build matrix. So I want to try that out. Also,
commit 8e31e4a7ea for issue #5 was intended to make this work on 5.2,
so, let's test for that version in particular.

Obviously I'm trying all this on a topic branch. May or may not merge
to master.