Travis: test with multiple versions of underscore and backbone

Related https://github.com/marionettejs/backbone.marionette/pull/2037

Fixes #167
Looks like we don't need this one given [the current deps](https://github.com/marionettejs/backbone.radio/blob/master/package.json#L43), but maybe whatever changes we need to make to support 0.9.9 will also let us support 0.9.0.