Swift 5 + CI integration

[ctreffs]

Great library! Thanks for creating and maintaining it!

This PR updates the package to Swift 5 amongst the following things:

• Migration to Swift 5
• Update Linux test manifest (may be auto generated with swift test --generate-linuxmain)
• Adds SwiftEnv support (makes it a lot easier to have a consistent dev environment locally)
• Adds Travis CI support (tests macOS + Linux)
• Adds Gitlab CI support (test Linux)
• Use canImport(<library>) over custom NOSIMD macro
• Fix some warnings
• Run SwiftLint

Hope you find it useful

[ctreffs]

CI runs revealed this error on Linux platforms: Vector2Tests.swift:27: error: Vector2fTests.testVectorMultiplyWithMatrix4x4 : XCTAssertEqual failed: ("1.0") is not equal to ("-1.0") +/- ("0.001")

See: https://travis-ci.com/ctreffs/SwiftMath/jobs/231230643