Update build configuration for the systemml website

Since a lot of dependencies have been improved, the present website build is failing.  updating the dependencies for the build configuration, such as 
  1. dependencies for the build (gulp)
  2. configuration file (task configuration)
 
1. Update `gulp` to `4.0.2`
link: gulpjs docs
 2. Upgrade npm dependencies
link: npm-check-updates
3. Stick with Susy 2. Because Susy 3 does not provide mixins.
related links
1. some mixins in susy are removed
2. susy media query handling