Release/6.0.0

[AliSoftware]

Checklist

• I've checked that all new and existing tests pass
• I've updated the documentation if necessary
• I've added an entry in the CHANGELOG to credit myself

Description

This branch contains the next release 6.0.0 of OHHTTPStubs.
This also closes #228 & #222.

Motivation and Context

This PR is to ensure the 6.0.0 release branch compiles correctly in all environments, by letting the CI matrix check them all, before officially making the release.

[Liquidsoul]

Ran some tests with Xcode 8.2 and everything seems to be working fine 👍

[AliSoftware]

@Liquidsoul 👌

I've added one last commit to add more information for Carthage users. I think that:

• given most people have migrated to Swift 3.0 already
• and that people still using Swift 2.3 won't be in a hurry migrating to OHHTTPStubs 6.0.0 if they haven't migrated to Swift 3.0 yet and those people will still use 5.2.3
• it's time-consuming to maintain a specific "swift-2.3" branch just for those users (which are likely not many, as projects in Swift 2.3 and using Carthage and not migrated to Swift 3.0 yet is rare by now)

Then I've decided to stop doing swift-2.3 specific branches just for so few people. Those people have alternatives anyway, and I don't want to lose time maintaining that old branch any longer for an obsoleted Swift version which is already not supported anymore in latest Xcode.

Most of Carthage users likely have already migrated to Swift 3.0 anyway so they'll be OK, and those who haven't are likely to stick with old versions of their dependencies anyway, including OHHTTPStubs 5.2.3, so it's probably not worth losing our maintainers time for this

[Liquidsoul]

Shouldn't the markdown badge [Language: Swift 3.1], even if not displayed, be updated to what the shield shows for consistency? (meaning Swift-2.3/3.0/3.1)

[AliSoftware]

Indeed!