Align sentiment-analysis' tokenizer (currently cased) to the model (uncased)
`sentiment-analysis` pipeline has a tokenizer using a cased vocab while the model is uncased. 

This PR makes the tokenizer uncased to match the actual model's vocab.
LGTM, or even better we could duplicate those tokenizer files into `distilbert-base-uncased-finetuned-sst-2-english` as now we kind of assume that a model == model + tokenizer.<|||||>Merging this for now and can update to @julien-c's proposal in a future PR<|||||>Yeah, just wanted to ping @mfuntowicz on this model/tokenizer subject <|||||>Hi guys, I pulled from master after the merge of the PR but the behaviour is still the same. It doesn't look like aligning the tokenizer resolved this issue.