Makes repo redirects work at 'opam init' and 'opam add'

they only worked on update -- hopefully this won't be too much of a problem for 1.1 users:
if they had a working install, all good, but otherwise they'll get a big bunch of warnings
(fixed by 'opam update')

Only workaround would be to keep the current location for the 1.1 repo and have 1.2 somewhere
else...