Don't round integer answers before validating them

Summary: Can you see a problem with this?

Test Plan:
http://localhost:8000/exercises/completing_the_square_1.html?debug&problem=original&seed=2&activity=%5B%5B%22incorrect-activity%22%2C%22%5B%5C%224.9%5C%22%2C%5C%22-6.9%5C%22%5D%22%2C3%5D%2C%5B%22correct-activity%22%2C%22%5B%5C%225%5C%22%2C%5C%22-7%5C%22%5D%22%2C3%5D%5D

http://localhost:8000/exercises/converting_decimals_to_fractions_2.html?debug&problem=into-mixed&seed=91&activity=%5B%5B%22incorrect-activity%22%2C%22%5C%229.29%5C%22%22%2C6%5D%2C%5B%22correct-activity%22%2C%22%5C%229%2029%2F100%5C%22%22%2C4%5D%5D

http://localhost:8000/exercises/converting_fractions_to_decimals.html?debug&problem=0&seed=25&activity=%5B%5B%22correct-activity%22%2C%22%5C%220.438%5C%22%22%2C8%5D%2C%5B%22correct-activity%22%2C%22%5C%220%2C438%5C%22%22%2C4%5D%5D

http://localhost:8000/exercises/multiplication_4.html?debug&problem=0&seed=111&activity=%5B%5B%22correct-activity%22%2C%22%5C%2241.976%5C%22%22%2C19%5D%2C%5B%22correct-activity%22%2C%22%5C%2241%2C976%5C%22%22%2C3%5D%2C%5B%22correct-activity%22%2C%22%5C%2241976%5C%22%22%2C1%5D%5D

Reviewers: alpert

Reviewed By: alpert

CC: peter

Differential Revision: http://phabricator.khanacademy.org/D2112