relax warning on ensure-contrast

...so we don't get bugged by our input:placeholder
styles

the number 2 was just something arbitrary that I had picked

Change-Id: I256880f0c4e3b11084387dfabf1dffa3fdaf6688
Reviewed-on: https://gerrit.instructure.com/38484
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Colleen Palmer <colleen@instructure.com>
Product-Review: Ryan Shaw <ryan@instructure.com>
QA-Review: Ryan Shaw <ryan@instructure.com>