CLC-4314 - Update gems: icalendar

https://jira.ets.berkeley.edu/jira/browse/CLC-4314

@ctweney @raydavis could you check the changelog in this case and see whether this impacts us?
https://github.com/icalendar/icalendar/blob/9268b2010893cd135958b6d5edbe4f0b801665cf/History.txt#L1

Especially "Default to non-strict parsing"
Merge pull request #3001 from christianv/CLC-4314

CLC-4314 - Update gems: icalendar