Fix sqlalchemy dependency

[Schaechtle]

We got a requirement by our industrial collaborator to bump the version of sqlalchemy. sqlalchemy is a dependency of CGPM. I tracked this down to CGPM through our build system (when I set the sqlalchemy version in our image itself -- Conda's sat solver found incompatibility with Bayeslite. When I set the version in Bayeslite Conda's sat solver found incompatibility CGPM. CGPM seems to be happy with an upgraded version.