Build: Change submodule CI updating

The clone operation on Travis will already update the submodules. The
checkout is all that is required to reset the headless repo in the
folder to the latest commit.
Merge pull request #2538 from pjackson28/v3.0

GCWU theme: Corrected header and footer links
Merge pull request #2539 from pjackson28/v3.0

Removed icons from documentation and question buttons in working examples
Merge pull request #2511 from nschonni/fix-submodule-updates

Build: Change submodule CI updating
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
Build: Localize build timestamp timezone

Use a daylight savings time aware timezone string, because Travis CI builds kept setting
the timestamp values to GMT
Fixes gh-2546
Added the timezone identifier
Merge pull request #2556 from LaurentGoderre/fix-build-tstamp-offset-2546

Fix build tstamp offset 2546
Updated dependencies for Lightbox, Tabbed Interface, Prettify and Slider Control.