Add JDK 13 to matrix build

[sfuhrm]

Update to newest install-jdk.sh from https://raw.githubusercontent.com/sormuras/bach/master/install-jdk.sh

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This is a small update of the Travis-CI setup to do a Matrix build with JDK 13.
It's best practice to fail-fast when a problem with new JDKs arises.

[mp911de]

Thanks for the PR. The bach script originates in the time where Travis had difficulties with newer JDK versions. I think we can switch to a more streamlined approach without hosting the script on our repo. Take a look at: https://github.com/spring-projects/spring-data-examples/blob/master/.travis.yml.

[sfuhrm]

@mp911de
There's one drawback with the wget option: When the remote repo has an instability/change, travis will fail here.
Anyway, I agree that it makes no sense to mirror something here that does not need to be mirrored.

[mp911de]

Thank you for your contribution. That's merged now.