Build failure with Java 11

I'm seeing a build failure with Java 11 on the master branch (below). It
would be nice to get this fixed up since Java 11 has been released for a
while now. Thanks!

[ERROR] Tests run: 2, Failures: 0, Errors: 2, Skipped: 0, Time elapsed:
0.166 s <<< FAILURE! - in
org.apache.cayenne.cache.invalidation.CacheInvalidationCacheGroupsHandlerIT

[ERROR]
invalidateE1(org.apache.cayenne.cache.invalidation.CacheInvalidationCacheGroupsHandlerIT)
Time elapsed: 0.081 s  <<< ERROR!

java.lang.NoSuchMethodError:
org.apache.cayenne.cache.invalidation.CacheInvalidationCacheGroupsHandlerIT.access$0(Lorg/apache/cayenne/cache/invalidation/CacheInvalidationCacheGroupsHandlerIT;)Ljava/util/concurrent/atomic/AtomicInteger;

at
org.apache.cayenne.cache.invalidation.CacheInvalidationCacheGroupsHandlerIT.invalidateE1(CacheInvalidationCacheGroupsHandlerIT.java:98)


[ERROR]
invalidateE2(org.apache.cayenne.cache.invalidation.CacheInvalidationCacheGroupsHandlerIT)
Time elapsed: 0.079 s  <<< ERROR!

java.lang.NoSuchMethodError:
org.apache.cayenne.cache.invalidation.CacheInvalidationCacheGroupsHandlerIT.access$1(Lorg/apache/cayenne/cache/invalidation/CacheInvalidationCacheGroupsHandlerIT;)Ljava/util/concurrent/atomic/AtomicInteger;

at
org.apache.cayenne.cache.invalidation.CacheInvalidationCacheGroupsHandlerIT.invalidateE2(CacheInvalidationCacheGroupsHandlerIT.java:135)