Do not return a -1 exit status after SIGINT
Do not return a -1 exit status after SIGINT

`signal.ExitStatus()` can return -1, which makes the program re-execute 
without the panic wrapper.

See https://golang.org/src/syscall/syscall_linux.go?s=5681:5717#L214

Closes #2
bugsnag/panicwrap