FIX filename json key

The json key use for files name was `file_name` whereas gist API uses
`filename` (without underscore) as a key. As a consequence download was
not working.
Change to use the right key.

REF #16

Details:
* CHANGE `file_name` to `filename`
Merge pull request #17 from MaximeD/hotfix-filename-json-key

FIX filename json key