Replace gem 'discordrb-webhooks-blackst0ne' with 'discordrb-webhooks'

This is a follow-up issue from gitlab-ce!22684

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22684 added new feature - Discord Notification service.

Unfortunately, I had to fork the original gem discordrb-webhooks and use my fork in the MR.
The reason was dependencies conflicts.

As it mentioned in the MR description, the upstream gem requires 'rest-client >= 2.1.0.rc1'

At the same time kubeclient gem, which we use, requires 'rest-client ~> 2.0'. That raises dependency conflicts.

In my fork I relaxed the dependency of rest-client.
Now it requires 'rest-client ~> 2.0' - the same version as kubeclient expects.

The dependency of rest-client >= 2.1.0.rc1 was added to fix some bugs for Ruby 2.5 on Windows OSes.

@jprovaznik suggested to push a simple PR to discordrb with if-checks for both platform and ruby version for rest-client dependency so we could use the original gem.

But @z64 (a maintainer of discordrb) said it wouldn't work like that and would require some more work.
Since the discordrb team works on possibly removing rest-client from their dependencies list, I think we should just wait for a new release and bump discordrb on our end.

Supporting a fork of actively developing project requires much effort.
So we should switch from the fork to the original discordrb gem as soon as possible.