Improve theme update script

When running composer drupal-theme-update we don't seem to get the composer object properly and hence, the variables are not defined. Turned out that was not the case. But here is the list of what still needs to be done:

• Re-enable gulp for composer install because that's required on live sites, check the sanity of the system first
• Re-build theme, including files that normally don't get overwritten
• Reset assets (node, bower, fonts, etc.) so that we get a clean state
• Add .gitkeep to .gitignore (is that the way to use this?)
• Using subdir img or images?
• Create _default_variables.scss and _bower_assets.scss only if required
• Post install may throw exception: npm WARN lifecycle undefined~postinstall: cannot run in wd %s %s (wd=%s) undefined find node_modules/ -name '*.info' -type f -delete /jails/lakedrops/var/www/web/themes/custom/lakedrops
• Run bower only if json file is available: bower ENOENT No bower.json present [Exception] Bower returned a non-zero exit code