Android 5.1.1 is coming soon for ZF2, and the most hilarious part, they're gonna start it from the lowest version (one with 720p) as confirmed by a Asus support. Here's the source link:-
Android 5.1 for Asus ZenFone 2 ZE550ML is in final testing - GSMArena.com news
Android 5.1 for Asus ZenFone 2 ZE550ML is in final testing - GSMArena.com news