travis: bump the Secure Transport build to use xcode 10

Due to an issue with travis
(https://github.com/travis-ci/travis-ci/issues/9956) we've been using
Xcode 9.2 for darwinssl builds for a while. Now xcode 10 is offered as
an alternative and as it builds curl+darwinssl fine that seems like a
better choice.

Closes #3062