Update Xcode versions used in CI to latest minor release #712
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Motivation and Context
Lately the ObjC tests that were using Xcode 10.1 on CI have been very unstable. It was not uncommon that two out of four CI runs failed. The Error is always the same and seems to be caused by a known defect in Xcode 10.1. The only known fix seems to be not to use 10.1. So that's what we will do.
Description
I've updated our Travis CI config to use Xcode 9.4 and 10.3 respectively. The update of Xcode 9 is not necessary, but since it's the current default for Travis CI ObjC images I thought it wouldn't hurt.
Testing
I've let the five CI jobs run four times each on my fork's CI with these changes and there were no errors.