MAINT: improve quad_form by avoiding square(sqrt(...))

This changes the `quad_form` implementation a bit so that https://github.com/cvxgrp/cvxpy/issues/45 will work with minimal modification (the redundant `<= 1` is removed).  All of the tests pass.  By the way, would it be worth adding a separate simplex constraint to cvxpy?  I'm not sure how to add this.