update version number and release date in changelog

Author: Kully

CHANGELOG.md

@@ -2,7 +2,7 @@
 All notable changes to this project will be documented in this file.
 This project adheres to [Semantic Versioning](http://semver.org/).
 
-## [2.3.1] - 
+## [2.4.0] - 2018-02-16
 ### Added
 - County Choropleth figure factory. Call `help(plotly.figure_factory.create_choropleth)` for examples and how to get started making choropleths of US counties with the Python API.