Make notebooks in examples run out of the box using Google colab (4/5)

[anupamamurthi]

• [ ] demo_optim_preproc_adult.ipynb
• [ ] demo_reject_option_classification.ipynb
• [ ] demo_reweighing_preproc.ipynb
• [ ] demo_short_gerryfair_test.ipynb

[fionavictoria]

Modified the four notebooks to run out of the box in google colab @anupamamurthi @nrkarthikeyan