Build on travis-ci
Rename informant submodule to core

This should help avoid confusion when referring to the 'informant'
folder (e.g. see README.md).
Switch back to Firefox for WebDriver tests

Seeing sporadic failures when using PhantomJS driver, see
ghostdriver issue #140.
Update to use new shaded artifact name
Travis-ci upload informant.jar to fixed location

And update README.md link to point to this fixed location.