Update changelog

Author: jonathanlloyd

CHANGELOG.md

@@ -5,6 +5,9 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/)
 and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
+### Added
+ - Support for `HTTP_PROXY` environment variables. This should enable use of the
+   SDK on the PythonAnywhere platform (see issue #24)
 
 ## [1.1.0] - 2019-02-06
 ### Added