bugfix: oracle-java8 is not available any more, but openjdk-8 works

Created by: muffato

Use case

Docker builds have broken recently, because apt can't install the Oracle's Java version. See https://cloud.docker.com/u/ensemblorg/repository/registry-1.docker.io/ensemblorg/ensembl-hive/builds/6ade0715-5644-474d-8799-e157bdbac273

Description

I've simply moved to openjdk. I can't remember why we needed oracle's version in the first place.

Possible Drawbacks

If anything requires Oracle specifically, but @mira13 is also leaning towards openjdk in #104

Testing

Have you added/modified unit tests to test the changes?

I have rebuilt the Docker image locally and ran the example Java pipeline with prove -v /repo/ensembl-hive/t/10.pipeconfig/guest_language.t

If so, do the tests pass/fail?

It worked

Have you run the entire test suite and no regression was detected?

Yes (within the Docker image)