[#1817] Upgrade SQLAlchemy to 0.9.x

No changes needed in the CKAN code base, but it requires changes on vdm,
see this branch:

https://github.com/okfn/vdm/tree/sqlalchemy-0.9-support

Also upgraded sqlalchemy-migrate.
[#1817] Update requirements file now that vdm is on pypi
Merge branch 'master' into 1817-upgrade-sqlalchemy

Conflicts:
	requirements.in
	requirements.txt
Merge branch '1817-upgrade-sqlalchemy'