catch a file that didn't make it into the Ensime support commit

The patch for the change that became sha 8313f22a89f06f4bf6595c4cc7f87bd0367cc17f
didn't apply cleanly. I tweaked it by hand, but a day later, I see
this modified file in my repo: the BUILD change to pull in the test.
Computering: it's difficult.

So here's the BUILD change. Good news: the test passes.

Testing Done:
https://travis-ci.org/pantsbuild/pants/builds/31726091

Reviewed at https://rbcommons.com/s/twitter/r/830/