corrected rounding error when creating custom grade scheme

floating point math error caused errors with certain percentages

fixes CNVS-1850

test plan:
- create/edit an assignment and go to manage grading schemes
- enter various percentages for grades (93.5, 87.6, 69.5, 69.6) and save
- grades should reflect the percentage entered after saving and not mutate

Change-Id: Ieaae9d5ea5f6436890794a029cf0a54749ccad18
Reviewed-on: https://gerrit.instructure.com/39662
Reviewed-by: Simon Williams <simon@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>