Require IndicatorJob.version in CREATE

Setting version in serializer validate method causes a bug
where the validate method gets called twice more than a
second apart, which then updates the version before posting
to geotrellis. Then, geotrellis can't find models with the invalid,
updated version.

Made this a required field on IndicatorJob since it will be necessary
once we update the version handling.
Where does this get set then?