Finish issue #4296 ("svn 1.8 blocks on release of serf 1.2.x").

(This will probably light up the buildbots...)

* INSTALL,
* configure.ac,
* get-deps.sh,
* tools/dev/windows-build/Makefile,
* tools/dev/unix-build/Makefile.svn
  Bump our Serf dependency to version 1.2.0.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1449186 13f79535-47bb-0310-9956-ffa450edef68