Added support for Service Roles to the EMR client.
Updated the CHANGELOG for release 2.6.8
Merge branch 'emr-updates'
Merge branch 'release-2.6.8'
Fixes #309. Ignores the new url_stat flag in PHP 5.5.13 when working with the S3 Stream Wrapper in order to preserve the current logic.
Updated CHANGELOG for 2.6.8 release.