Avoid problem with colons in output of svnversion.

`svnversion' output can contain colons. If this happens, then the following Makefile line is misparsed by GNU Make:

$(DRAFTS): %-$(REVISION).pdf: %.pdf

The error is: "Makefile.include:136: *** target pattern contains no `%'.  Stop."

Avoid this error by replacing colons in the output of `svnversion' with dashes.