Replaced try/finally with autoreleasepool as a way to eat the @.

As reported in ReactiveCocoa/ReactiveCocoa#952 the presence of a `@try` block silences the warning about control reaching the end of a non-void block or function.

This PR replaces `@try` with `@autorelease` as a way to eat the @.

Both libextobjc's and RAC's tests pass with this change.