Bump to version 4.0.0

• New version numbering (redmine_gitlab_hook v4.0.0 is for Redmine v4.0.x).
• Add new API key check inherited from Redmine SysController (Thx to @chainboy).
• Add Redmine 4.0.x support (/pull/48, Thx to @Dkstu).
• Add Windows server support (/pull/44, Thx to @ghost).

Bump to version 0.2.2

• See /issues/40.
• See /issues/41.
• See /issues/27.
• See /issues/38.

Bump to version 0.2.1

• See /issues/38.

Bump to version 0.2.0

• See /pull/36.
• See /pull/37.
• Many thx to @jcppkkk and @3kami3 for the pull request.