Monkeypatch FactoryOptions for backwards compatibility

[Matoking]

Fixes #47

factory_boy 2.9.0+ renames the 'postgen_declarations' parameter in FactoryOptions to 'post_declarations', and uses a DeclarationSet instead of a dict to handle the declarations.

This pull request monkey patches the newer FactoryOptions instances to include a 'postgen_declarations' property, which should behave similarly enough to the attribute in the older factory_boy versions.

pytest_factory's own unit tests don't pass with the branch as-is, but exposing the 'postgen_declarations' allowed the test suite for my Django web application to work again. factory_boy has gone through a lot of refactoring, so I doubt a complete fix would be nearly as simple.

The solution still feels quite hacky to me, so I'm up for a better solution if one is created. One option is dropping support for older factory_boy versions, though that would involve fixing the library to work with the newer factory_boy.

---

This change is 

[olegpidsadnyi]

I'll work on the major release