Remove Subversion from prerequesite lists

As far as I can tell, keeping SVN is a historical accident at this point. 
It's not actually needed for the build, and it's a non-trivial prerequisite 
if it is just a forgotten package. I haven't tested all features of ZM yet, 
so perhaps I'm very mistaken about this! I just did a grep for "svn" and 
"subversion" and noticed no usage, removed it, and the build passes