[racket] daily package builds and tests

I've uploaded a preview of package-build results using the v6.1.1
release candidate:

 http://next-pkg-build.racket-lang.org.s3-website-us-west-2.amazonaws.com/

You'll see that the build ran `raco test --drdr` for each successfully
installed package, and the web page reports whether any tests failed.
(After the v6.1.1 release, those test runs will become part of the
regular package-build service.)

For more information about how tests are run, follow the "About Package
Builds" link at the top of the page.