Support for older Xcode 8.2.1

[aonez]

Continuing from #6, also had to change some code in order to compile the framework, due the fact that I'm using Xcode 8.2.1 (8C1002), old Mac with 10.11 El Capitan, old Swift libraries.

Don't know if you want to make the framework compatible with legacy versions of Swift. No changes needed on current Xcode 8.3.3 👍

Here the changes I've made. Know that just made them quick so the compiler let me try the framework.

[nfgrilo]

Thank you!

However, as I have just converted ThemeKit to Swift 4 (1.1.0), Xcode 9 is now a requirement, so I will close this ticket as is. Your changes can still be very useful for anyone using ThemeKit 1.0.0 (which uses Swift 3.2) + Xcode 8 👍

Again, thanks!