Twitter-server 18.12.0

Twitter-oss: Prepare OSS libraries for release 18.12.0

Problem

We want to release the next versions of our Twitter OSS libraries 18.12.0
 - util
 - scrooge
 - finagle
 - twitter-server
 - finatra

Solution

Prepare libraries for their next releases.

Differential Revision: https://phabricator.twitter.biz/D251041