update icalendar for ruby 2 support

Change-Id: I3249cc0516c83e22b94c9f25c3ae86b5468ee23e
Reviewed-on: https://gerrit.instructure.com/36744
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Nick Cloward <ncloward@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>