message: Add release date to changelog
  Should have been included in
diff: ## [Unreleased]
  -## [1.1.0]
  +## [1.1.0] - 2020-04-22
  ### Changed
  - [browser/node] Build process updates. Bumping minor version for this. See
  [#646](https://github.com/airbrake/airbrake-js/pull/646)
  