iOS 9 visibility fix.
iOS 9 visibility fix.

Hello!
`canOpenURL:` works differently under iOS 9. And prevents this activity from appearing even if no x-callback is set. 
A workaround (for now) is to not check availability of x-callback URLS if  `callbackURL` is not set. 
This allows the activity to be usable (just without x-callback).

Under iOS9 there might be a better option with universal links but I think that is on chrome for iOS's side of things to expose.
alextrob/ARChromeActivity,morizotter/ARChromeActivity