message-switch: add upstream version contraints
message-switch: add upstream version contraints

Now the opam file of message-switch is identical to that in its source
repo (at the HEAD of its master branch).

Signed-off-by: Gabor Igloi <7535f5501fd71876c93d896691e178fd3a11cfbb@citrix.com>

gaborigloi/xs-opam,xapi-project/xs-opam,lindig/xs-opam,mseri/xs-opam,mseri/xs-opam