Fix Main Thread issue

[jguffey]

Swift 5 support

• Changes compile language to Swift 5
• Changes iOS build target to 12.4
• Compiles and runs in Swift5

Thread Issues

8577da7 Resolves #19

SNFilter is subclass of UIImageView, which can only be init'd from the main thread.

You'd have to not be a UIView if you want to do background threading.

Now, changing to generate filters on the main thread, has no perceivable impact on filter generation on my iPhoneX, nor iPhone6.