update minimum supported safari, chrome and firefox

test-plan:
* with chrome 31 & 32, verify browser warning does not display
* with safari 6 & 7, verify browser warning does not display
* with firefox 26 & 27, verify browser warning does not display

Change-Id: I8b5250908bfb0269d3616fbf06a5e6da65e016c3
Reviewed-on: https://gerrit.instructure.com/28717
Reviewed-by: Bryan Madsen <bryan@instructure.com>
Product-Review: Bryan Madsen <bryan@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>