Update SwiftPM Dependency

Merge pull request #4 from llvm-swift/bump

Update to Swift 4.2

Merge pull request #3 from llvm-swift/bump

Update to Swift 4.1

Try not to use-after-free

Merge pull request #2 from llvm-swift/numerology

Add numbering to stack dump entries

Make sure to free the stack trace C string when popped

Merge pull request #1 from llvm-swift/signal-safety-dance

Make library more signal safe