OF-1383: Have CI use both Java 8 and 11.
OF-1383: Have CI use both Java 8 and 11.

This commit causes Travis to compile the project not only with Java 8, but also with Java 11.

speedy01/Openfire,igniterealtime/Openfire,GregDThomas/Openfire,GregDThomas/Openfire,guusdk/Openfire,speedy01/Openfire,speedy01/Openfire,igniterealtime/Openfire,igniterealtime/Openfire,guusdk/Openfire,akrherz/Openfire,igniterealtime/Openfire,GregDThomas/Openfire,GregDThomas/Openfire,akrherz/Openfire,guusdk/Openfire,GregDThomas/Openfire,speedy01/Openfire,akrherz/Openfire,igniterealtime/Openfire,guusdk/Openfire,guusdk/Openfire,speedy01/Openfire,akrherz/Openfire,akrherz/Openfire