Switch (back) to using the head-of-stream JaCoCo.

Switch (back) to using the head-of-stream JaCoCo.

When I upgraded from Java 7 to Java 8, JaCoCo broke.
At that time, the most recent version of the Maven Plugin did not work,
and I had to hard-code a (slightly older) version in order to get builds to work.

Now, I have upgraded to Java 9, and the previously-hard-coded version does not work with that.
Removing the version specifyer (well, commenting it out) brings me back to a state where
builds succeed.