Adapt to signature change of org-icalendar--combine-files

(org-caldav-generate-ics): Signature of `org-icalendar--combine-files'
changed with version 8.2.8 of Org.
Merge commit 'f5c0e2728abd8959456f5798a6811f015ebccece'

Conflicts:
	org-caldav.el