Support Swift 4.2

[djones6]

Description

Updates for Swift 4.2 to resolve compilation warnings. Specifically:

• Removes the Swift 3 format Package.swift, since this package no longer supports Swift 3
• Updates Package.swift to Swift 4.2 format, and renames the Package@Swift-4.x.swift -> Package@swift-4.x.swift (SPM otherwise does not pick these up),
• Updates .swift-version to 4.2 and adjusts .travis.yml accordingly,
• Defines the OpenSSL dependency as an embedded system library for 4.2,
• Copies the module.modulemap and shim.h files from OpenSSL into a new target in this package,
• Adds a errToString(ptr: UnsafeMutablePointer<Int8>?) -> String helper to avoid string interpolation warnings with optionals.

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)

Also resolves warnings due to string interpolation of UnsafeMutablePointer? in several error logging messages, which I suspect is related to the rework of implicitly unwrapped optionals: https://github.com/apple/swift-evolution/blob/master/proposals/0054-abolish-iuo.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.