Swift 4.1 support release

[dmcyk]

Hey there 👋

Unfortunately the latest version fails build with Xcode 9.3 (Swift 4.1), though that's been already fixed by @danj-stripe and on master the build succeeds.
Are you planning on doing any other work before releasing a new version with the fix?

Thanks :)

[danj-stripe]

I also started #185, but didn't know what was necessary to complete the release. Maybe keep an eye on that PR?

[dmcyk]

oh right, sorry, should have checked that first. I will close this and follow the PR, thanks