The Travis build is currently failing, and I suspect this is why. Double.NaN isn't equal to anything, including itself.

The fix for this is to use Hamcrest matchers, and the `notANumber()` matcher that is in `lenskit-test`.  So this becomes:

```java
assertThat(similarity.similarity(emptyVector(), emptyVector()),
           notANumber());
```

with `assertThat` coming from `org.junit.Asserts`.