Prepare v1.3.0 (#98)

Add a changelog entry with a planned release on Monday.

Author: akshayjshah

CHANGELOG.md

@@ -4,6 +4,10 @@ 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/)
 and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.html).
 
+## 1.3.0 - 2018-07-30
+### Added
+- Add a `RawSource` option that selectively disables variable expansion.
+
 ## 1.2.2 - 2018-07-12
 ### Changed
 - Undo deprecation of `NewProviderGroup`.