update xsum
This PR makes the following update to the xsum dataset:

- Manual download is not required anymore

- dataset can be loaded as follow: `nlp.load_dataset('xsum')`


**Important** 
Instead of using on outdated url to download the data:  "https://raw.githubusercontent.com/EdinburghNLP/XSum/master/XSum-Dataset/XSum-TRAINING-DEV-TEST-SPLIT-90-5-5.json" 

a more up-to-date url stored here: https://s3.amazonaws.com/datasets.huggingface.co/summarization/xsum.tar.gz is used
, so that the user does not need to manually download the data anymore. 
There might be slight breaking changes here for xsum. 