Summary:
Pull Request resolved: https://github.com/facebook/Ax/pull/507

As discussed with Balandat offline yesterday, we should pin BoTorch versions so that we don't always have to release Ax right after BoTorch.

Right now, we're in a slightly dangerous state where BoTorch 0.4.0 is released, and so because our setup.py file runs `pip install botorch>=0.3.3`, pip will install 0.4.0 now. But the stable version of Ax is actually not compatible with this newest version of BoTorch (as you can see from the failure of this test: https://github.com/facebook/Ax/runs/1967381927?check_suite_focus=true).

Reviewed By: lena-kashtelyan

Differential Revision: D26635518

fbshipit-source-id: bd34718a9faf55f3dbc0bf6115a96b72f4468372