Downgraded TensorFlow to 1.5 to fix AVX incompatibility

[jcohenadad]

Older CPUs do not support AVX, which is a requirement for TF>1.5. Several users (>5) are not able to use SCT because of that, so this PR downgrades TF to 1.5 until we find another solution (e.g., move to Pytorch).

Also removed requirements-freeze.txt (it was mistakenly added to master branch, but should only be present in release)

Fixes #2562

[bcdarwin]

Looks fine to me; installation, sct_check_dependencies, and sct_testing all succeed with the correct downgraded TF/Keras versions. I haven't had a chance to try retraining the models, though.

[jcohenadad]

Thanks @bcdarwin! Before merging I'd like to get feedback from at least one user in this situation to make sure it works on their end. Users contacted:

• Illegal instruction when importing Keras/Tensorflow with CPU not supporting AVX #2562 (comment)
• http://forum.spinalcordmri.org/t/installation-failed-during-installation-of-keras/349/5?u=jcohenadad

UPDATED 2020-03-02 10:16:36: worked for one of the user above:
http://forum.spinalcordmri.org/t/installation-failed-during-installation-of-keras/349/9?u=jcohenadad

[jcohenadad]

Since this is a big deal, I am also testing all the scripts from the SCT course...

UPDATE 2020-03-02 11:52:15: Everything went fine 👌