Keep the exact amount, rather than random delta

https://github.com/sunitparekh/data-anonymization/blob/master/lib/strategy/field/number/random_integer_delta.rb
https://github.com/sunitparekh/data-anonymization/blob/master/lib/strategy/field/default_anon.rb
Merge pull request #177 from alphagov/whitelist-settled_amount

Keep the exact amount, rather than random delta