use https for rubygems source

prevents a possible MITM attack on installed code

Change-Id: I13c4419eb3f469c2f895a8725b18d90e35ccc992
Reviewed-on: https://gerrit.instructure.com/30753
Reviewed-by: Brian Palmer <brianp@instructure.com>
QA-Review: Shawn Meredith <shawn@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>