We can make use of https://github.com/moliware/travis-solr to aid us in our test coverage on travis.

Here's a run:
https://travis-ci.org/andrewnicols/moodle/jobs/150843033#L418

Doing this adds about 30-60 seconds to the run time of the build so we may not want to do it on all PHPUNIT runs (5.6 PgSQL, 7.0 MySQL, 7.0 PgSQL).
Adding a separate build just for solr will take longer than the time saved by not adding it to all three.

So question is do we want to run it on all three, or just a sub-selection?