Update travis builds to xcode 11.3.
Update travis builds to xcode 11.3.

Bump the base image for macOS travis builds to XCode 11.3 on
macOS 10.14. This have more up-to-date software than the
default image, which is conservative.

The downside is that this version will need to be bumped periodically.

xiph/opusfile,xiph/opusfile,xiph/opusfile