Prevent slope of 0 in point slope form

Summary: If the slope is 0 then infinitely many equations are obviously right (with different x-values) and I think it's confusing to only count two of them as correct, so disallow it.

Test Plan: Loaded a problem of each type locally.

Reviewers: eater

Reviewed By: eater

Subscribers: jessie

Differential Revision: http://phabricator.khanacademy.org/D11548