Support Swift 4.2

[djones6]

Description

Updates for Swift 4.2 to resolve compilation warnings. Specifically:

• Updates .swift-version to 4.2 and adjusts .travis.yml accordingly,
• Reorganizes the Package.swift files:
  • Removes a redundant Package.swift, and renames the 4.2-compatible [email protected] to Package.swift,
  • Updates Package.swift to Swift 4.2 format (// swift-tools-version:4.2) so that we build in 'full' 4.2 mode,
  • Renames the [email protected] -> [email protected] (SPM otherwise does not pick these up)
• Updates the Swift 4.2 package to use version 2.0 of the OpenSSL dependency, which has been restructured into Swift 4.2 format,
• Adds a missing dependency declaration for OpenSSL to the CryptorRSA target to eliminate a warning (I'm surprised this compiled before).

I noted that there is another compile warning relating to returning a force-unwrapped type. I did not attempt to address this as part of this PR.

Motivation and Context

Resolves compiler warnings relating to the use of system packages with Swift 4.2.
(see: https://github.com/apple/swift-evolution/blob/master/proposals/0208-package-manager-system-library-targets.md)

How Has This Been Tested?

Checklist:

• I have submitted a CLA form
• If applicable, I have updated the documentation accordingly.
• If applicable, I have added tests to cover my changes.