require sqlalchemy 0.8

We are relying on SQLAlchemy 0.8 features now. Right now it's just the
new sqlalchemy.sql.expression.{true,false} functions, but we will also
be merging Alembic support soon and that requires 0.8 as well.

Change-Id: I76c1f7ab43fb7a6aeecad2322f9eea2d4bd2b7f6