Use github versioning
I noticed that datasets like `cnn_dailymail` need the `version` parameter to be passed to its `config_kwargs`.
Shall we rename the `version` paramater in `load_dataset` ? Maybe `repo_version` or `script_version` ?
Right now dataset scripts and metrics are downloaded from S3 which is in sync with master. It means that it's not currently possible to pin the dataset/metric script version.

To fix that I changed the download url from S3 to github, and adding a `version` parameter in `load_dataset` and `load_metric` to pin a certain version of the lib, as in #562 