Swift 5 conversion

[TomCarey]

Used Xcode 10.2 (10E125) to convert to Swift 5 automatically.
Applied recommended project changes.

[GeroHerkenrath]

Hey, @TomCarey!

Thanks for starting this. I began the same task for Ambassador, see envoy/Ambassador#48. It seems I might have found an issue that would also bite your migration should you fix/update the travis config for this PR to run the tests successfully, see https://forums.developer.apple.com/thread/113919.

In short: String(cString: UnsafePointer(strerror(errno))) will most likely have to be changed to just String(cString: strerror(errno))

[TomCarey]

@GeroHerkenrath No need to have two swift 5 PRs.. I'll close my one.