Swift 5.1 Support

### New Issue Checklist

- [x] I have tried with the latest version of OHHTTPStubs
- [x] I have read the [README](https://github.com/AliSoftware/OHHTTPStubs/blob/master/README.md)
- [x] I have read the [Using the right Swift Version of `OHHTTPStubs` for your project](https://github.com/AliSoftware/OHHTTPStubs#using-the-right-swift-version-of-ohhttpstubs-for-your-project) section
- [x] I have searched in the [existing issues](https://github.com/AliSoftware/OHHTTPStubs/issues?utf8=✓&q=is%3Aissue)
- [x] I have read [the OHHTTPStubs wiki](https://github.com/AliSoftware/OHHTTPStubs/wiki) to see if there wasn't a detailed page talking about my issue

### Environment

- version of OHHTTPStubs: 6.2.0
- integration method you are using:
* [x ] Cocoapods
* [ ] Carthage
* [ ] submodule
* [ ] other
- version of the tool you use: 1.7.5

### Issue Description

Xcode and Swift 5,1 have both been shipped recently and it would be great to get support added for Swift 5.1.

The project that I am working on using OHHTTPStubs is working in Swift5.0 and Xcode 10.3 but not in Xcode 11 and Swift 5.1