Resolve Swift 4.2 compile warnings

[djones6]

Resolves #198

As well as resolving a compile warning relating to changes to force-unwrapped optionals in Swift 4.2, this PR:

• Updates the swift-tools-version in Package.swift to 4.2, in order to compile in 'full' Swift 4.2 language mode
• Updates .swift-version to 4.2, and adjusts .travis.yml accordingly,
• Adds a Package@swift-4.0.swift and Package@swift-4.1.swift for backward compatibility.

FYI, warnings in dependencies are being addressed under these PRs:

• https://github.com/IBM-Swift/Kitura/pull/1338 and https://github.com/IBM-Swift/Kitura/pull/1340 (tagged in Kitura 2.5.3)
• https://github.com/IBM-Swift/Kitura-net/pull/277 (tagged in Kitura-net 2.1.2)
• https://github.com/IBM-Swift/BlueSSLService/pull/57 (tagged in BlueSSLService 1.0.18)
• https://github.com/IBM-Swift/BlueCryptor/pull/46 (awaiting review)

[djones6]

@hhellyer @sjanuary Please could someone review? (I don't have permission to request one)

[djones6]

@hhellyer @tobespc Could we get this reviewed and tagged if you're happy? Thanks