Bug when getting model from project version

[joaomarcoscrs]

For some projects versions, the model is being None when it should not be.

This happens because the API may return a single 'model' key, not always 'models' on the check below

https://github.com/roboflow/roboflow-python/blob/3620fda4b84bd495ec2a2a2ebbfad6e7951189c6/roboflow/core/version.py#L100

[iurisilvio]

It was a recent change. Ref #276.

cc @stellasphere