Swift 4 migration

[webframp]

Swift 4 bring some necessary migrations for the existing code.

Primarily the implicit objc inference behavior is removed:

• https://stackoverflow.com/questions/44379348/the-use-of-swift-3-objc-inference-in-swift-4-mode-is-deprecated
• https://github.com/apple/swift-evolution/blob/master/proposals/0160-objc-inference.md

anyone is welcome to work on this, just let me know so we don't duplicate effort.

[webframp]

more work is still needed

[webframp]

Waiting on https://github.com/dropbox/SwiftyDropbox/issues/173

[michaeleverson]

I've been working on this one. Please have a look when you get a chance. #234

Are we wanting to go to 4.2?

[michaeleverson]

I've also removed the old style String use, but feel this would need move testing as the implications of editing this are bigger. #235