Add ability to get ranking type performed.

Return the type of ranking the API performed, from the attune-* response
headers in the get_rankings and multi_get_rankings calls.

Some possible values are
* attune-ranking [indicates what ranking was performed]
  * default
  * attune
  * attune-experimental
* attune-cell [indicates which cell the user is in]
  * control
  * attune
  * intermediate
Update major version for latest changes.

Latest breaking changes to get_rankings and multi_get_rankings reason a major
version bump.