Increase tariff memory threshold.
Increase tariff memory threshold.

Tariff frontend currently levels off at aroung 2.7G. This adjusts the
memory thresholds accordingly.

This also updates how the number is specified to be more obvious.  The
previous value worked out to 2684354560 which wasn't obvious from
reading the source.

alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet