Less aggressive formula question variable caching

Seems like the problem was that the cached data would not get
invalidated in the case of generation failure, causing any further
changes (or recomputations) to the variables to be ignored.

Closes CNVS-13702

TEST PLAN
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See Ian's screencast in the ticket's comments:
(https://instructure.atlassian.net/secure/attachment/21167/CNVS-13702.swf)

That should no longer be reproducible. Instead, it should generate some
solutions when you do change the numbers.

  - go to new quiz page
  - add a formula question, and do things in exactly this order:
    1. write "[a] - [b]" in the description
    2. fill in the first variable [a] with 1 and 1 for Min and Max
    3. fill in the second variable [b] with 1 and 1 for Min and Max
    4. fill in a formula with "a-b" then click Save
    5. fill in the "Offer" field with like 10
    6. press Generate
  - you should now receive an error alert
  - now do this:
    1. change [b] values to 3 and 3 for Min and Max
    2. press Recompute
    3. press Generate again
  - you should no longer see the error alert, and it should generate the
    possible solutions

Change-Id: I1f79e3196439b109322a697075265a7b15b13aab
Reviewed-on: https://gerrit.instructure.com/41681
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Ryan Taylor <rtaylor@instructure.com>
Reviewed-by: Derek DeVries <ddevries@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Product-Review: Derek DeVries <ddevries@instructure.com>