Update less version to 1.5 to match webapp

Summary: Mostly updating to get rid of the log, but also consistency; that's important.

Test Plan: Open up browser, make sure entire perseus css not broken. Also, open up console and ensure that the log is gone

Reviewers: alpert

Reviewed By: alpert

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D6806

Auditors: jack, eater, justinjaffray