[DEP] Switch to icalendar HEAD.
[DEP] Switch to icalendar HEAD.

We submitted a bugfix for a performance issue upstream over three weeks
ago and they still haven't cut a new release though they said they'd do
it that week.

Fixes T1967

gale320/sync-engine,gale320/sync-engine,ErinCall/sync-engine,wakermahmud/sync-engine,ErinCall/sync-engine,Eagles2F/sync-engine,gale320/sync-engine,nylas/sync-engine,wakermahmud/sync-engine,Eagles2F/sync-engine,nylas/sync-engine,closeio/nylas,gale320/sync-engine,Eagles2F/sync-engine,ErinCall/sync-engine,jobscore/sync-engine,nylas/sync-engine,wakermahmud/sync-engine,closeio/nylas,ErinCall/sync-engine,jobscore/sync-engine,jobscore/sync-engine,ErinCall/sync-engine,nylas/sync-engine,closeio/nylas,gale320/sync-engine,Eagles2F/sync-engine,wakermahmud/sync-engine,jobscore/sync-engine,wakermahmud/sync-engine,Eagles2F/sync-engine,closeio/nylas