nil out the previous backtrace when a backtrace is deallocated
Remove overridden -dealloc method
Don't use GCD's QSS destructor

Since backtraces only need to be retained for a single scope anyways,
this is more deterministic and simpler.
Merge pull request #741 from ReactiveCocoa/backtrace-lifetime-fix

Fix for RACBacktrace lifetime crash
Merge branch 'master' into 2.0-development