(Waiting for next PsyNet release) Fix script for updating demo and test experiments

Update demos/update_demos.py to correctly update the requirements.txt and constraints.txt files

requirements.txt:

• On release branches: use the to-be-released version, e.g. psynet==13.0.0
• On the master branch: use the master branch, i.e. psynet@git+https://gitlab.com/PsyNetDev/PsyNet@master#egg=psynet