GitLab compatibility with Ruby 2.4

I noticed we started using yield_self in the code base. This is the method from Ruby 2.5 and I believe it makes our application incompatible with Ruby 2.4. However I could not find anywhere us announcing that we don't support Ruby 2.4 anymore.

Few examples of using yield_self in the code:

• https://gitlab.com/gitlab-org/gitlab-ee/commit/5aa5681e48950f8747b985caa12ba7c8c40c6a81#1e4cd4835b112afcebfe2cbcc49a6ed56fe85c16_16_16
• https://gitlab.com/gitlab-org/gitlab-ee/commit/c86015bf335a#note_138767218