update minimum supported browsers

fixes #CNVS-12517

test-plan:
* with chrome 33 & 34, verify browser warning does not display
* with safari 6 & 7, verify browser warning does not display
* with firefox 27 & 28, verify browser warning does not display
* with internet explorer 10 & 11, verify browser warning does not display

Change-Id: I8a86dcef641c8c2b936097237e239320e6b0190c
Reviewed-on: https://gerrit.instructure.com/33493
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jon Jensen <jon@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Bryan Madsen <bryan@instructure.com>