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

CHANGELOG.rst

@@ -7,6 +7,9 @@ Note that ``PHAB_ID=#`` and ``RB_ID=#`` correspond to associated messages in com
 Unreleased
 ----------
 
+18.12.0
+-------
+
 New Features
 ~~~~~~~~~~~~
 

README.md

@@ -1,6 +1,6 @@
 # Twitter Util
 
-[![Build status](https://travis-ci.org/twitter/util.svg?branch=develop)](https://travis-ci.org/twitter/util)
+[![Build status](https://travis-ci.org/twitter/util.svg?branch=master)](https://travis-ci.org/twitter/util)
 [![Codecov](https://codecov.io/gh/twitter/util/branch/develop/graph/badge.svg)](https://codecov.io/gh/twitter/util)
 [![Project status](https://img.shields.io/badge/status-active-brightgreen.svg)](#status)
 [![Gitter](https://badges.gitter.im/twitter/finagle.svg)](https://gitter.im/twitter/finagle?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge)
@@ -35,7 +35,7 @@ for more details about how to contribute.
 An example SBT dependency string for the `util-collection` tools would look like this:
 
 ```scala
-val collUtils = "com.twitter" %% "util-collection" % "18.11.0"
+val collUtils = "com.twitter" %% "util-collection" % "18.12.0"
 ```
 
 # Units

build.sbt

@@ -1,7 +1,7 @@
 import scoverage.ScoverageKeys
 
 // All Twitter library releases are date versioned as YY.MM.patch
-val releaseVersion = "18.12.0-SNAPSHOT"
+val releaseVersion = "18.12.0"
 
 val zkVersion = "3.5.0-alpha"
 val zkClientVersion = "0.0.81"