Travis CI: test across multiple versions of Neo4j!

By downloading and installing locally. Thanks @wfreeman for the example!

https://github.com/wfreeman/mvcs/blob/master/.travis.yml

Fixes #110.
Revert "Travis CI: debug-log Neo4j version."

This reverts commit f38370d7977b91b83e0254d14767038fdf45c64e.

No longer needed since we now control the Neo4j version.