Prepare for release of v2.3.1 (#937)

* Prepare for release of v2.3.1

* Add ts3.4 to files list in package.json

* Update version numbers and package-lock

Co-authored-by: Justin Fagnani <[email protected]>

Author: rictic

CHANGELOG.md

@@ -11,11 +11,17 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 -->
 
 <!-- ## [x.y.z] - YYYY-MM-DD -->
+<!-- ## Unreleased -->
 <!-- ### Changed -->
 <!-- ### Added -->
 <!-- ### Removed -->
 <!-- ### Fixed -->
 
+## [2.3.1] - 2020-03-19
+
+### Fixed
+* Add TypeScript type declarations for older versions of TypeScript. We're currently testing back to TS 3.4. We can't commit to never breaking TypeScript builds, but we'll be supporting older versions as best we can.
+
 ## [2.3.0] - 2020-03-18
 
 ### Changed