WP Unit Tests, Travis, and other improvements

* Fix WP Unit Test problem Fixes #34 
* Update `.travis.yml` to use same `WP_VERSION` as template from WP-CLI
* Update `pre-commit` hook to natively support invoking `phpunit` on VVV.
* Fetch and use latest install-wp-tests.sh from wp-cli GitHub repo.
* Add support for `.ci-env.sh` to override default `.travis.yml` settings. Implements part of #30.