DownloadPsychtoolbox, GetSubversionPath: Match svn client path search order.

It was different between DownloadPsychtoolbox and the rest, which could
cause funnyness if multiple svn clients were installed on the same OSX
machine in the different locations, e.g., if the clients required different
svn repository formats and caused "upgrade your repo" errors.

Closes issue #103 on Psychtoolbox main repo.