update ucontext factory to download the archive instead of using svn; use only one buildslave, used in two builders (I begin to understand buildbot philosophy, I think) git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/simgrid/simgrid/trunk@6023 48e7efb5-ca39-0410-a469-dd3cf9ba447f