Feature/gulp migration

[m-suchorski]

Gulp migration.
This should fix a building error that occurs with current build of gulp for some reason.

Motivation and Context

PR for @tkaik

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.

---

I hereby agree to the terms of the AET Contributor License Agreement.

[malaskowski]

Travis build fails on:

[ERROR] 
[ERROR] events.js:183
[ERROR]       throw er; // Unhandled 'error' event
[ERROR]       ^
[ERROR] Error: assets/sass/main.scss
[ERROR] Error: File to import not found or unreadable: ../../assets/libs/bootstrap-sass-twbs/assets/stylesheets/_bootstrap.scss.
[ERROR]         on line 18 of assets/sass/main.scss
[ERROR] >> @import "../../assets/libs/bootstrap-sass-twbs/assets/stylesheets/_bootstrap
[ERROR]    ^
[ERROR] 
[ERROR]     at options.error (/home/travis/build/Cognifide/aet/report/src/main/webapp/node_modules/node-sass/lib/index.js:291:26)

It looks that this file is missing, should it be downloaded automatically during build phase?