Don't accept decimals for the 'integer' form

Summary: Closes #39522. :(

Test Plan:
Verified converting_decimals_to_fractions_2 does not take a decimal, that
converting_fractions_to_decimals does, and that area_of_squares_and_rectangles
still works with integer answers.

Reviewers: eater

Reviewed By: eater

CC: cbhl

Differential Revision: http://phabricator.khanacademy.org/D2023