Pin factory-boy dependency on 1.3.x branch

Factory Boy 2.0 have removed the "extracted_key" keyword argument from
post_generation() hook. Until we can update tests to work with 2.0 make
dependency on 1.3.x branch. In turn, the update got stalled by a bug
in Factory Boy (https://github.com/rbarrois/factory_boy/issues/53).