Removed warnings found in Swift 4.1

[KyeMaloy97]

There are a low level API changes in Swift 4.1 which were giving warnings in other projects that depended on this repo, such as Kitura Websocket.

This PR just adds some conditional checks to use the relevant API depending on the Swift version.

Ran the test suite locally and built locally too on Swift 3.1.1, 4.0.3 and 4.1. I have added Travis builds for these Swift versions too.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@7e24e6b). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master      #34   +/-   ##
=========================================
  Coverage          ?   76.72%           
=========================================
  Files             ?       10           
  Lines             ?      954           
  Branches          ?        0           
=========================================
  Hits              ?      732           
  Misses            ?      222           
  Partials          ?        0

Flag	Coverage Δ
#Cryptor	76.72% <100%> (?)

Impacted Files	Coverage Δ
Sources/Cryptor/StreamCryptor.swift	81% <100%> (ø)
Sources/Cryptor/HMAC.swift	70.86% <100%> (ø)
Sources/Cryptor/Digest.swift	93.18% <100%> (ø)

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[ianpartridge]

You checked in the Xcode project.... Please remove it and add Cryptor.xcodeproj to .gitignore.

[ianpartridge]

I don't think this line should be inside the #if block?

[ianpartridge]

Hang on, I'm an idiot. I didn't realise this project has an Xcode project checked in already :(

In that case, I don't think we should touch it in this PR. @KyeMaloy97 can you remove the change to .gitignore and Cryptor.xcodeproj.

Just change .swift-version, Sources and .travis.yml in this PR.

Sorry.

[billabt]

This PR looks fine to me. Will merge it. An' yes, it has an Xcode project. This is because there are variations on the build for different platforms that have been requested by users.

[billabt]

This looks fine as is with the Xcode project.