Update to fifth release
Mostly a bug fix update for loading the `TransfoXLModel` from s3:
- this fixes a bug in the loading of the pretrained `TransfoXLModel` from the s3 dump (which is a converted `TransfoXLLMHeadModel`) and the weights were not loaded.
- I also added a fallback of `OpenAIGPTTokenizer` on BERT's `BasicTokenizer` when SpaCy and ftfy are not installed. Using BERT's `BasicTokenizer` instead of SpaCy should be fine in most cases as long as you have a relatively clean input (SpaCy+ftfy were included to exactly reproduce the paper's pre-processing steps on the Toronto Book Corpus) and this also let us use the `never_split` option to avoid splitting special tokens like `[CLS], [SEP]...` which is easier than adding the tokens after tokenization.
- I also updated the README on the tokenizers options and methods which was lagging behind a bit.