[DEPENDENCIES] Upgrade to SQLAlchemy 0.9.6.

Summary:
The proximal reason for this is to take advantage of the new load_only feature
(http://docs.sqlalchemy.org/en/rel_0_9/changelog/migration_09.html#new-query-options-api-load-only-option),
which we could employ for great gains in performance.

I've been running this version for a bit locally with no obvious breakages, and
don't see anything in the changelog that would affect us.

Test Plan: Unit tests and local syncing.

Reviewers: spang

Reviewed By: spang

Differential Revision: https://review.inboxapp.com/D204