Updating versions for next release

I am proposing:

• tensorflow: 2.0.0
• keras: 2.3.1
• torch: 1.3.0.post2
• theano: 1.0.4
• scikit-learn: 0.21.3
• numpy: 1.17.3
• pandas: 0.25.2
• networkx: 2.4
• pyarrow: 0.15.1
• scipy: 1.3.1