[racket] Scribble change, especially affects `scribble/sigplan'

As of v5.1.0.5, Scribble renders `racketblock', `codeblock',
`interactions', etc., in a different way than before. Indentation
relative to surrounding text is now implemented by a nested-flow style,
instead of adding spaces to the start of each line. (Using spaces was a
hack from before sorting out various rendering problems with nested
code blocks.)

For most Scribble documents, you shouldn't notice the difference, but
`scribble/sigplan' mode for Latex/PDF output is an exception. The
Scribble style adjustments for the SIGPLAN format now introduce
vertical space before and after a code block. The change is intended to
make uses of `racketblock', `codeblock', etc., look better by default,
but the change could make existing documents look worse.

To disable the extra space (because you don't like it or because
you've already added space in some other way), add these commands in
your document's Latex configuration:

 \renewcommand{\SCodePreSkip}{}
 \renewcommand{\SCodePostSkip}{}