Travis: Don’t specify JDK version for OSX build
Travis: Don’t specify JDK version for OSX build

Even when we specify "jdk: openjdk11" for OSX build, Travis seems to be
ignoring that and instead just using JDK13. For our CI purposes, we
don’t really care which JDK version Travis uses for OSX builds anyway,
so let’s not specify a particular JDK version for OSX.

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