ME1 validation should not include export_refund_nomenclature_sid

Documentation states:
The combination of
measure type +
geographical area +
goods nomenclature item id +
additional code type +
additional code +
order number +
reduction indicator +
start date

must be unique.
Merge pull request #202 from alphagov/ME1

ME1 validation should not include export_refund_nomenclature_sid