Update tracis cache config as recommned by the docs (#3575)
Update tracis cache config as recommned by the docs (#3575)

https://docs.travis-ci.com/user/languages/java/#Caching
JabRef/jabref,zellerdev/jabref,JabRef/jabref,sauliusg/jabref,Siedlerchr/jabref,zellerdev/jabref,sauliusg/jabref,JabRef/jabref,Siedlerchr/jabref,zellerdev/jabref,JabRef/jabref,tobiasdiez/jabref,sauliusg/jabref,tobiasdiez/jabref,tobiasdiez/jabref,sauliusg/jabref,Siedlerchr/jabref,tobiasdiez/jabref,zellerdev/jabref,zellerdev/jabref,Siedlerchr/jabref