Migrate to Xcode 10.2 and Swift 5

[dterzic]

• Migrated to Xcode 10.2 and Swift 5.
• Fixed warnings reported by Swift 5.

[GeroHerkenrath]

Hm, this is surprising, I would have expected travis to complain about return String(cString: UnsafePointer(strerror(errno))), as that throws an error when I try to build Ambassador on it (see envoy/Ambassador#48).

Guess that's because naturally that pipeline still pulls the Swift 4.2 version of the code.

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Note that this PR here is already a step further than #70 as the only difference so far seems to be that you also updated the travis configuration, so I am curious where the warnings should be fixed. I'd be willing to contribute (I took a look at the warnings locally already), but I don't wanna open yet another PR. What's the plan for now, if there is any?

[dterzic]

@GeroHerkenrath Yeah, I saw that PR but it had some build errors.
Because we needed Embassy in order to complete the migration of Kiosk I just added another (temporary) branch (didn't even fixed the warnings).
@LFabien What do you think is the best way to proceed?

[LFabien]

@dterzic @TomCarey @GeroHerkenrath Thanks for working on that, I'll merge this PR.

@GeroHerkenrath I think this error has been fixed here #66