Update to v0.20.0 of Swift SDK

[glennrfisher]

This pull request updates the text-to-speech sample app to use v0.20.0 of the Swift SDK.

[mkistler]

I'm curious as to why there are no changes to the xcodeproj. Don't we need to explicitly add Starscream to the build?

[glennrfisher]

Not in this case, as it turns out. We still don't support WebSockets for Text to Speech—only Speech to Text. So this sample app for Text to Speech doesn't depend upon Starscream.

So why update it, then? I wanted to get the latest version in the Cartfile.resolved. We tend to suggest that everyone runs carthage update --platform iOS. But we should really be telling them to run carthage bootstrap --platform iOS for the sample apps.

(The update command will find the latest version that satisfies any version constraints in Cartfile. But the bootstrap command downloads the same version as in Cartfile.resolved.)

By suggesting bootstrap instead, we can make sure that people are using the same version of the Swift SDK and dependencies that we tested with. But it means we need to keep Cartfile.resolved up-to-date.

[mkistler]

Looks good! 👍