Fix macos CI

[justin1121]

There's a problem with the xcode version on circleci not being compatible with the tensorflow build I did for macos. Figure out how we can make these work together.

[justin1121]

To reproduce the root cause: ssh into the circleci container and run:

cd repo
. venv/bin/activate
python -c "import tensorflow"

It will crash with Illegal Instruction.

[justin1121]

If we continue using a custom version of Tensorflow we'll most likely have to build tensorflow on a machine provided by circleci. This was the suggestion from circleci support.