Require java 8. It was require in docs for a long time.
Require java 8. It was require in docs for a long time.

TODO cleanup guava functional backports.

Signed-off-by: Kanstantsin Shautsou <726516dea2238e859d5028bfb21f227f655fd603@gmail.com>

jenkinsci/github-integration-plugin,jenkinsci/github-pullrequest-plugin,KostyaSha/github-integration-plugin,jenkinsci/github-integration-plugin,jenkinsci/github-pullrequest-plugin,jenkinsci/github-pullrequest-plugin,jenkinsci/github-integration-plugin,KostyaSha/github-integration-plugin,KostyaSha/github-integration-plugin,KostyaSha/github-integration-plugin