Fix Travis CI to use xcode image 11.3 or greater.

[RobertGardner]

In .travis.yml the iOS builds use osx_image: xcode10. This should be updated to xcode11.3 (or 11.4 when it's available). (And remove the -env field for Xcode since it should be redundant?)

Note that currently if that change is made, the builds fail with:

The command sudo pip install six exited with 1.
Log:
sudo: pip: command not found
/Users/travis/.travis/functions: line 607:  3574 Terminated: 15          travis_jigger "${!}" "${timeout}" "${cmd[@]}"
The command "travis_wait sudo pip install six" failed and exited with 1 during .