Adapt to API of aeson 0.6.2.0.

The API of `deriveJSON` changed despite 0.6.2.0 only being a minor bump.  The
used options:

    defaultOptions
        { sumEncoding = ObjectWithSingleField
        , allNullaryToStringTag = False
        }

Are backwards compatible with using `id`  in 0.6.1.0.

Signed-off-by: Magnus Therning <magnus@therning.org>