allow ruby master to run with scary warning

sometimes it's needed to debug stuff in 2.1, cause it
fixes SystemStackError

bundler doesn't support it yet, so we have to use conditionals
instead of platform options

Change-Id: I88614634d24f36d7c18c999c1dc65eb91df65a32
Reviewed-on: https://gerrit.instructure.com/37010
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jon Jensen <jon@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>