round mastery gradebook scores to two decimal places

fixes CNVS-13528

test plan
- create some outcome results with fractional scores (live
 assessments are great for this)
- ensure that the scores on the mastery gradebook rounds to two
 decimal places
- ensure that the header (which computes an average) also rounds

Change-Id: Icfa91e0a1d56afe3be2c73aabe32188fb52ba2ef
Reviewed-on: https://gerrit.instructure.com/36202
Reviewed-by: Braden Anderson <braden@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Product-Review: Joel Hough <joel@instructure.com>