Use unsafe_unretained to allow a target of 4.3
Use unsafe_unretained to allow a target of 4.3

This is the default for Unity, so it would be nice to just work.

joshdholtz/bugsnag-cocoa,joshdholtz/bugsnag-cocoa,joshdholtz/bugsnag-cocoa,bugsnag/bugsnag-cocoa,bugsnag/bugsnag-cocoa,joshdholtz/bugsnag-cocoa,joshdholtz/bugsnag-cocoa,bugsnag/bugsnag-cocoa,bugsnag/bugsnag-cocoa