Upgrade JUnit

[pdurbin]

I just realized that there's no Javadoc for the version of JUnit we're using (4.8.1): http://search.maven.org/#search|gav|1|g%3A%22junit%22%20AND%20a%3A%22junit%22

It looks like I added that version a loooong time ago in 66dd604 so I doubt anyone care which version we upgrade to as long as it works.

[pdurbin]

Meh, we can live without Javadoc for Junit. It's pretty straightforward.

[pdurbin]

Reopening and assigning to @poikilotherm as discussed at http://irclog.iq.harvard.edu/dataverse/2018-09-19 and moving to "Community Dev" at https://waffle.io/IQSS/dataverse . Thanks!!