Require a particular commit of SayIt, not master

As Matthew Somerville pointed out:

   "Just to note that I don't think the requirement commit should be
    moved back to master; this will immediately break any newly deployed
    Pombola as soon as an incompatible change is made in SayIt (e.g. all
    the redesign changes just merged today). I think the branch name
    should be maintained in SayIt and it can then choose to point at
    which ever commit will maintain working status."

I'm not sure it's worth still keeping the "pombola" branch in the
SayIt repository, though, since that might still cause surprise if
someone updates that branch without thinking about the consequences
for the next deploy - if we just require a particular commit, changes
to the version should (!) always be more thoughtful.