I am the author of the smeserver-subversion contrib, this contrib only integrates subversion with SME Server, subversion is taken from the repositories where SME Server is taking it's packages from, including upstream (CentOS) repositories.
AFAIK 1.4.6 is the last release able to run on and released for SME Server 7.x (and CentOS 4.x) because of dependencies for higher versions not being available for SME Server 7.x (and CentOS 4.x).
For SME Server 8.x you are able to use a more recent version, where 1.6.11 is the latest available AFAICT.