Build: Init submodule for Travis demo building

Use the already cloned wet-boew-dist to save the extra full clone. There
are notes on using the --reference command on
https://www.kernel.org/pub/software/scm/git/docs/git-clone.html, but
those shouldn't be affected by this usage.
Merge pull request #2540 from nschonni/init-submodules-for-demo-updates

Build: Init submodule for Travis demo building