allow key ':index' in native 'update-aliases'.

The REST client expects the key ':index' while the native client uses
':indices'. This causes undesired effects (in particular: the adding of
an alias to _all_ indices) when using ':index' with the native client
(see #108).

This change allows ':index' while maintaining backwards-compatibility by
merging ':index' and ':indices'.
update ChangeLog.md.