Support Swift versions 3.1 through 4.1

[djones6]

Changes to allow PathKit to build on Swift 3.1, 4.0 and 4.1:

• Support Swift 3.1 through 4.1 by reintroducing the Swift 3 format Package.swift from 0.8.0, and renaming the Swift 4 format to Package@swift-4.swift
• Remove deprecation warnings for String API changes in Swift 4 - similar to Removing String's deprecated APIs and modifying for Swift 4 #42 / Removed character reference for Swift 4.0.3 support #45, but guarded to maintain compatibility with Swift 3
• Address compilation error on Linux + Swift 4.1 due to ObjCBool: Add boolValue property swiftlang/swift-corelibs-foundation#1223
• Update lastComponentWithoutExtension test for Linux + Swift 4.1
• Add travis matrix for testing with both Swift 3.1.1 and 4.0 snapshots (I didn't add a 4.1 dev snapshot, but this would be straightforward)

[ianpartridge]

LGTM - @kylef?

[ianpartridge]

@kylef ping.. we need Swift 4.1 support here otherwise we'll have to do a temporary fork :-(

[keith]

We've just release 0.9.1 which includes #48. This has all the same fixes here, except we opted to not support 3.1. Thanks for submitting this though!