Update requirements to prevent gensim breaking

[tyronechen]

Dear authors, thank you for the useful package. With updates to gensim, scipy and scikit-learn, the old requirements.txt file no longer works. This pull request resolves that.

In the original requirements.txt file, gensim >=3.4 was specified. After the release of gensim >=4, API changes prevented code from running. The new requirements file fixes gensim at 3.4.

Errors were obtained when running with some versions of scikit-learn and scipy. scikit-learn==0.18.2 and scipy==1.2.0 were tested and found to work.

MulticoreTSNE==0.1 was also added to requirements file.

Finally, existing installs of certifi may cause problems. This can be fixed by specifying --ignore-install certifi to pip install. README.md was updated with this note.