Use es6-promise package to avoid webpack warnings

What does this MR do?

Use the es6-promise package to avoid the webpack warning fluff, Module not found: Error: Can't resolve 'vertx'

See https://github.com/stefanpenner/es6-promise/issues/100

-> https://github.com/stefanpenner/es6-promise/blob/f5f44e81e3d92db1445d61c157a3593a3662ef81/package.json#L54

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Before

bundle exec rspec /Users/eric/Documents/gitlab/gitlab-development-kit/gitlab/spec/features/merge_requests/create_new_mr_spec.rb:95
Run options: include {:locations=>{"./spec/features/merge_requests/create_new_mr_spec.rb"=>[95]}}
[WDS] Warnings while compiling.                                                                                       |  ETA: ??:??:??
./vendor/assets/javascripts/es6-promise.auto.js
Module not found: Error: Can't resolve 'vertx' in '/Users/eric/Documents/gitlab/gitlab-development-kit/gitlab/vendor/assets/javascripts'
 @ ./vendor/assets/javascripts/es6-promise.auto.js 140:16-26
 @ ./app/assets/javascripts/application.js
 @ multi (webpack)-dev-server/client?http://localhost:3808 ./application.js
[WDS] Warnings while compiling.
./vendor/assets/javascripts/es6-promise.auto.js
Module not found: Error: Can't resolve 'vertx' in '/Users/eric/Documents/gitlab/gitlab-development-kit/gitlab/vendor/assets/javascripts'
 @ ./vendor/assets/javascripts/es6-promise.auto.js 140:16-26
 @ ./app/assets/javascripts/application.js
 @ multi (webpack)-dev-server/client?http://localhost:3808 ./application.js
[WDS] Warnings while compiling.
./vendor/assets/javascripts/es6-promise.auto.js
Module not found: Error: Can't resolve 'vertx' in '/Users/eric/Documents/gitlab/gitlab-development-kit/gitlab/vendor/assets/javascripts'
 @ ./vendor/assets/javascripts/es6-promise.auto.js 140:16-26
 @ ./app/assets/javascripts/application.js
 @ multi (webpack)-dev-server/client?http://localhost:3808 ./application.js
[WDS] Warnings while compiling.
./vendor/assets/javascripts/es6-promise.auto.js
Module not found: Error: Can't resolve 'vertx' in '/Users/eric/Documents/gitlab/gitlab-development-kit/gitlab/vendor/assets/javascripts'
 @ ./vendor/assets/javascripts/es6-promise.auto.js 140:16-26
 @ ./app/assets/javascripts/application.js
 @ multi (webpack)-dev-server/client?http://localhost:3808 ./application.js
 1/1 |===================================================== 100 =====================================================>| Time: 00:00:09

Finished in 9.21 seconds (files took 11.02 seconds to load)
1 example, 0 failures

After

bundle exec rspec /Users/eric/Documents/gitlab/gitlab-development-kit/gitlab/spec/features/merge_requests/create_new_mr_spec.rb:95
Run options: include {:locations=>{"./spec/features/merge_requests/create_new_mr_spec.rb"=>[95]}}
 1/1 |===================================================== 100 =====================================================>| Time: 00:00:08

Finished in 8.05 seconds (files took 12.55 seconds to load)
1 example, 0 failures

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Tests
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

n/a