travis: Check that modules don't break when dependencies aren't installed

[JDGrimes]

When a module for WordPoints integrates with another plugin, that module should not break the site even if that other plugin is not installed. In addition, some parts of the module should probably continue to work, like the uninstall routine. So that should probably be checked as well.

Eventually this will be better handled by WordPoints itself, but until then we might want to consider testing this.