Travis builds need to use tagged release of java-models-library

[tautschnig]

The update in diffblue/java-models-library#5 broke the jbmc/coreModels test. @smowton identified the culprit and guided me in crafting #2415, but such surprise breakage should be avoided in future. Thus java-models-library should provide tagged releases that Travis builds fetch.

[tautschnig]

This was fixed in #2426 by using a git submodule instead of downloading.