Got Javanica unit tests compiling (but failing)

I still see errors of the form : 
```java.lang.NoSuchMethodError: java.util.concurrent.ConcurrentHashMap.keySet()Ljava/util/concurrent/ConcurrentHashMap$KeySetView```.

This looks related to a Java7/Java8 mismatch described here: https://gist.github.com/AlainODea/1375759b8720a3f9f094

cc @sawano in case you've seen this before