Update facebook.rb

Removed reference to no longer existing field 'username' in the Graph API v2.0. See https://developers.facebook.com/docs/apps/changelog#v2_0_graph_api which states "/me/username is no longer available". Other possibility would be to set 'nickname' => raw_info['id'] for backward compatibility.

Requesting "/me?fields=username" (https://developers.facebook.com/tools/explorer?method=GET&path=me%3Ffields%3Dusername&version=v2.0) yields in an error: "(#12) username is deprecated for versions v2.0 and higher"