Fix pain points in GPT-2 tutorial

[mapmeld]

• WikiExtractor repo recently changed things around - simpler to use previous script by checkout old commit hash

• Added pip install line to one of the GPT-2 tutorial notebooks (I think it's your main one). Recently pip install fastai2 alone installs fastcore 1.x, leaving a confusing error; adding a specific pip install fastcore==0.1.38 resolves this