Bump version to 6.1.9
Bump version to 6.1.9-dev
Upgrade Aspen to pick up zetaweb/aspen#153
Bump version to 6.1.10
Bump version to 6.1.10-dev
Update opt_in to use new exceptions; #535

@joonas changed Participant.change_id to throw custom exceptions instead
of depending on Response and IntegrityError. This updates a case where
we catch those.
Bump version to 6.1.11
Bump version to 6.1.11-dev