Bump version to 3.1-stable \o/

Congratulations to everyone in the Godot community for this awesome new
release, culmination of more than one year of development from close to
500 contributors!

Thanks to all involved, whether you contributed code, documentation,
bug reports, translations, community support or donations. You all
played a role in bringing better free and open source game development
tools to the world!

Godot 3.1 includes more than 7000 commits made since the 3.0 release in
January 2018, 3000 Pull Requests have been merged, and 3000 issues have
been fixed!

This release makes the 3.x branch more stable and powerful, and makes
it a very mature game development tool for both 2D and 3D.

Now feature development can restart towards 3.2 and 4.0!

CHANGELOG.md

@@ -4,7 +4,7 @@ All notable changes to this project will be documented in this file.
 
 The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 
-## [Unreleased]
+## [3.1] - 2019-03-13
 
 ### Added
 

version.py

@@ -2,6 +2,6 @@ short_name = "godot"
 name = "Godot Engine"
 major = 3
 minor = 1
-status = "rc"
+status = "stable"
 module_config = ""
 year = 2019