[racket] No v5.3 release

Dear Racketeers,

As we are preparing the v5.3 release, we notice unexplored corners
of new and revised features. Two concerns stand out: the addition of
sub-modules and the extensive injection of contracts into the class
hierarchy in our run-time library. In short, our changes aren't
ready for an actual release.

Since we consider none of the changes critical for any of our users,
we have decided to skip the May release date. Instead, we will
have our next release in August (skipping the version number 5.3).

If this delay causes you any inconvenience, please let us know.

For those users who wish to take advantage of the new features,
please use the nightly builds available from
http://pre.plt-scheme.org. Doing so will help us discover problems
and will help you transition to the new release early.

Robby, on behalf of the Racket developers