Bump sqlalchemy-migrate version to 0.9.1

The latest version of sqlalchemy-migrate has support for sqlalchemy 0.9
and DB2.  Several core projects are supporting DB2 already so we should
bump the runtime requirement so the migrations work if you're running
against a DB2 backend.

This is also needed for oslo.db commit 567175554 to clean old migrate
monkey patches out of oslo.db since they live in migrate 0.9.1 now.

Related blueprint db2-database

Change-Id: Iadb4a5dd67d7e4e5547b7185159ab687a4ef1b74
Merge "Bump sqlalchemy-migrate version to 0.9.1"