Replace custom BankersRounding with upgraded Money gem

Money now handles bankers rounding by default, so it will apply to all
calculations that use it. Combined with the update to Ruby 1.9.3,
there's no longer a need to use a custom class to work around a bug with
the rounding.
Merge pull request #173 from alphagov/bankers-rounding

Replace custom BankersRounding with upgraded Money gem