No longer accept answers with a negative denominator
Merge pull request #43528 from petercollingridge/non-neg-denom

No longer accept answers with a negative denominator

fix #43485; and keep Alpert happy.

Test plan: Test exercises http://localhost:8000/exercises/simplifying_rational_expressions_2.html and make sure it still accepts the correct answer whilst rejecting answers with negated denominator and numerator.

Auditors: alpert