It seems that AC team had already launched 3.3.6 but they have said that 3.3.5 is the first stable release so I think it can be used in Prod environment.
activeCollab 3.3.6
Enhancements:
System checks for Phar extension before auto-upgrade and provides user friendly error message
Only non-archived projects are displayed on projects timeline
Bug Fixes:
Fixed missing Invoices Report icon when Tracking module was not installed
Removed apc.stats check from environment validation before installation
Fixed disk usage value casting bug
Fixed issue with inline images in global documents
Fixed issue where quotes were not properly upgraded in some cases
First stable activeCollab 3.3 release. Major new features:
activeCollab can upgrade itself
Great new overview tools - projects timeline and workload report
A lot of invoicing improvements - multi-line invoice items, support for second tax and compound taxing, amount due in invoice PDF-s, improved currency formatting, automatic invoice overdue reminder with message escalation and more
Invite people dialog
Faster navigation thanks to the improved main menu
Disk utility helps you manage disk usage