Add `model_version` field to /predictions endpoint

[Haimchen]

Goal

Add a new field model_version to the /predict endpoint.

Description

To evaluate the quality of the model later, the feedback about mislabeled tweets should contain the version of the model that made the (wrong) prediction.
As we do not know the model version yet, we will initally hardcode it to always return 1.

Don't forget to update the README 😉