Use 'cvs version' instead of 'cvs rls' to test remote repositories.

'cvs rls' is only available in CVS 1.12+, which isn't yet considered stable.
Fortunately, if 'cvs version' is run with a CVSROOT, it will end up checking
whether that CVSROOT is valid, which is perfect for our needs.

Testing done: Ran unit tests.

Fixes bug 3343.

Reviewed at https://reviews.reviewboard.org/r/5831/