Remove gulp package config

Gulp (1) is not used anymore and has been replaced with Webpack (2) or
by simply using scripts or npm scripts (3). Therefore the gulp package
config (4) can be removed. This improves the shell startup time since
the auto-completion script of gulp is not sourced anymore.

References:

  (1) https://gulpjs.com
  (2) https://webpack.js.org
  (3) https://docs.npmjs.com/misc/scripts
  (4) https://github.com/arcticicestudio/igloo/blob/c1fdba9e6554a4c1cdff32618ddafd700bc1a4cc/snowblocks/bash/config/pkg/gulp

Closes GH-145