Upgrade to Swift 4

[jeffaburt]

After merging this PR, please create a 2.0.0 tag:

cd HidingNavigationBar/
git checkout master
git pull origin master
git tag 2.0.0
git push origin 2.0.0

Then, send an update to CocoaPods, that way we can just run pod update HidingNavigationBar and get all of the Swift 4 goodies:

pod trunk push

Thanks!

[tristanhimmelman]

Thank you very much for handling the migration 👍