Revert "Merge pull request #123 from alphagov/bankers_rounding"

This reverts commit 7ac0b2b4aa41146837a53959bdd34c17c4baba76, reversing
changes made to e084bb5adb2cfa6b4563471f75a43f84efbda6b7.

The merged code broke the build, so undoing will ask for a fix on the
relevant branch.