Fix versions of dependencies in requirements.txt

[adocherty]

Currently installing a new environment and dependencies with the requirements.txt file causes a lot of warning messages introduces with numpy 1.17. While we may not want to restrict libraries to a single version in the main library just due to warnings, it would be advantageous to do so in the course.

I'd propose something like (I haven't tested this):

stellargraph[demos]==0.7.1
numpy==1.16.1
pandas==0.25.0
ipykernel
scikit-learn==0.21.3
seaborn==0.9.0
tensorflow==1.14.0