Fix the serve-precommit target.

When the plovr-jar is being invoked in the serve-precommit target, it needs
the JSON files in `test/` as well to have all requirements for headless testing.

This should also reenable running the testsuite on our continuous integration
server.
Merge pull request #189 from marcjansen/travis-tests

Fix the serve-precommit target. (r=@elemoine/@twpayne)