I don't know when Hadoop picked up Mockito, but it has been frozen at 1.8.5 since the switch to maven in 2011.
Mockito is now at version 2.1, with lots of Java 8 support. That' s not just defining actions as closures, but in supporting Optional types, mocking methods in interfaces, etc.
It's only used for testing, and, provided there aren't regressions, cost of upgrade is low. The good news: test tools usually come with good test coverage. The bad: mockito does go deep into java bytecodes.