Support Swift 5

[djones6]

Update to Swift 5, and use official Docker images for CI.

WIP: Tests fail to compile on MacOS with Swift 5:

/Users/travis/build/IBM-Swift/CircuitBreaker/Tests/CircuitBreakerTests/CircuitBreakerTests.swift:129:32: error: 'defaultRunLoopMode' has been renamed to 'RunLoop.Mode.default'
    RunLoop.current.run(mode: .defaultRunLoopMode, before: Date(timeIntervalSinceNow: 0.001))
                               ^~~~~~~~~~~~~~~~~~
                               RunLoop.Mode.default
Foundation.RunLoop.Mode:4:23: note: 'defaultRunLoopMode' was obsoleted in Swift 4.2
    public static let defaultRunLoopMode: RunLoop.Mode
                      ^

Also, there are lots of intermittent failures, not the same set captured in #49 but I suspect the same underlying problem. This really needs investigation.