bump libxml-ruby for ruby 2.2 support

Change-Id: I89cdb646dd6b519d5fb10d31004432a39957b06f
Reviewed-on: https://gerrit.instructure.com/46321
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Rob Orton <rob@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>