Corrected rounding function when letter grade is computed

A rounding error on 10 pt scale was causing 0.87 to display as a B instead of B+

fixes CNVS-11843

test plan:
- Create assignment worth 10 total points and score it by letter grade
- Use default grading scheme so B+ is >90% - 87%
- Save the assignment and as the test student submit to it
- In gradebook score it with an 8.7/10, should show B+ as grade
- Update score to 8.69/10, should show B as grade

Change-Id: I1c8abbaa53c49eb019cb19798ddc232bf65f9056
Reviewed-on: https://gerrit.instructure.com/38759
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Mike Nomitch <mnomitch@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>