fix the yahoo configuration

yahoo provider extends ouath1 provider and ouath1 provider uses the index key not the index id.
see the following links for more info.
https://github.com/hybridauth/hybridauth/blob/master/hybridauth/Hybrid/Providers/Yahoo.php#L20
https://github.com/hybridauth/hybridauth/blob/master/hybridauth/Hybrid/Provider_Model_OAuth1.php#L67
Merge pull request #319 from mahmouds/patch-3

fix the yahoo configuration