Swift 5 / Xcode 11 support [WIP]

[jakunico]

I had an old project that depended on reddift - I'm trying to resurrect that project and reddift was not working with swift 5 and Xcode 11. This PR should hopefully fix that.

App, frameworks and tests compile with no warnings:

I've faced a couple of challenges with tests:

1. Some tests are failing to me, from what I saw some of them depend on your reddit user, for instance UsersTest.testGetUserContents uses sonson_twit username which obviously does not match mine (see image below).
2. Travis needs to read some encrypted variables which are not available when you are a contributor (as you would be able to edit the travis script and print out those). Maybe you can merge temporarily into a swift-5 branch instead of master and then run tests on travis by opening a swift-5 > master PR.