round scores to 2 decimals for ember quiz moderate

in the event a score gets recorded as a repeating fraction, round the
display number for ember quiz moderate display

closes CNVS-13543

test plan:
  - enable :quiz_stats and :quiz_moderate features
  - create a quiz that could result in a fractional answer
    - i.e. a fill in mulitiple blanks (or add fractional fudge points)
  - as student take the quiz
  - as instructor visit ember moderate
    - verify that fractional answer is rounded to two decimal places
    - verfiy that none fractional numbers continue to show without
      decimals

Change-Id: If7bc567d440b75985f09e355c52379999366f2ba
Reviewed-on: https://gerrit.instructure.com/36170
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Derek DeVries <ddevries@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Derek DeVries <ddevries@instructure.com>