Enable testing of Oracle on TravisCI

[koppor]

This refs JabRef#1703

Quoting JabRef#1451 (comment)

Yes, they are no longer available at the public maven repo, but Oracle has now its own repo. https://blogs.oracle.com/dev2dev/entry/how_to_get_oracle_jdbc
However, it requires an account. I think @koppor should register that for jabref:
http://docs.oracle.com/middleware/1213/core/MAVEN/config_maven_repo.htm#MAVEN9012

Currently, lib/ojdbc.jar is automatically used during compile time if it is available.

To enable testing on TravisCI, following to issues have to be solved:

• Store ojdbc.jar encrypted on https://files.jabref.org/, fetch it at build preparation and decrypt it --> Include Oracle's JDBC driver directly JabRef/jabref#5517
• Download Oracle Database 11g Express Edition and enable it on Travis. It is "free to develop, deploy, and distribute" and thus, it might be stored unencrypted on files.jabref.org.

[koppor]

Orcales JDBC drivers are now available on maven central: https://medium.com/@kuassimensah/oracle-jdbc-drivers-on-maven-central-64fcf724d8b

[koppor]

For the second point, maybe custom runners help: https://github.blog/2019-11-05-self-hosted-runners-for-github-actions-is-now-in-beta/