message: Updated CHANGELOG for 1.5.1 release.
diff: # Changelog
  ## Unreleased
  -- Upgraded web-component-tester to v6.1.2.
  <!-- Add new, unreleased items here. -->
  +## v1.5.1 [08-22-2017]
  +- Upgraded web-component-tester to v6.1.2.
  +
  ## v1.5.0 [08-22-2017]
  - Fix issue where the `--fragment` flag was being ignored.
  - Added support for `polymer test --npm` option.
  