Fix the deprecation warnings (UIDocumentPickerViewController)

[w-i-n-s]

Also, if you're bumping the iOs version, let's also update the recommended project settings and fix the deprecation warnings.

Originally posted by @Sjors in #44 (comment)

[Sjors]

The deprecation messages are generating a lot of noise in the Travis log: https://travis-ci.com/github/Sjors/nthkey-ios/builds/234515720