Bump the asset-manager CPU warning threshold
Bump the asset-manager CPU warning threshold

As when asset-manager is busy, the value seems to be ~330. It seems to
be working fine when under load, so up the threshold a bit.

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