compatibility with 5.2

[technomancy]

I get this when trying to require rackjure under 5.2.1, which is what ships with Debian stable:

default-load-handler: cannot open input file: "/usr/share/racket/collects/racket/format.rkt" (No such file or directory; errno=2)

If you decide to only target 5.3+ it might be worth mentioning in the readme.