Parse formula question values to float when sorting for median

Fixes CNVS-1630

Test plan:
- Add a formula question to a quiz.
- Add a median formula with 3+ values.
- Generate a large number of possible combinations.
- Check all of these to verify that the correct number is given as the median.

Change-Id: I909ed89cb26be248b0fa012822735fd032c1e577
Reviewed-on: https://gerrit.instructure.com/43541
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Ryan Taylor <rtaylor@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Product-Review: Derek DeVries <ddevries@instructure.com>