Twitter-oss: update OSS libraries post-release to 21.1.0

Problem

We want to update to the next SNAPSHOT version of our Twitter OSS libraries 21.1.0
 - util
 - scrooge
 - finagle
 - twitter-server
 - finatra

Solution

Prepare libraries for their next SNAPSHOT version.

JIRA Issues: CSL-10389

Differential Revision: https://phabricator.twitter.biz/D591241

README.md

@@ -1,6 +1,6 @@
 # TwitterServer
 
-[![Build status](https://travis-ci.com/twitter/twitter-server.svg?branch=release)](https://travis-ci.com/twitter/twitter-server)
+[![Build status](https://travis-ci.com/twitter/twitter-server.svg?branch=develop)](https://travis-ci.com/twitter/twitter-server)
 [![Codecov](https://codecov.io/gh/twitter/twitter-server/branch/develop/graph/badge.svg)](https://codecov.io/gh/twitter/twitter-server)
 [![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)

build.sbt

@@ -1,7 +1,7 @@
 import scoverage.ScoverageKeys
 
 // All Twitter library releases are date versioned as YY.MM.patch
-val releaseVersion = "20.12.0"
+val releaseVersion = "21.1.0-SNAPSHOT"
 
 val jacksonVersion = "2.11.2"
 val jacksonLibs = Seq(