update ruby-saml-mod for canvas

fixes: CNVS-11530

See PR: https://github.com/bracken/ruby-saml/pull/20

Test Plan:

Make sure SAML is not broken.
Make sure Signature element is not included in logout requests.

Change-Id: I9d17378136eb2cce4353731c90ab909bad0a54ca
Reviewed-on: https://gerrit.instructure.com/32558
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Reviewed-by: Brian Palmer <brianp@instructure.com>
Product-Review: Nick Cloward <ncloward@instructure.com>