Build(deps): Bump onebox from 2.1.4 to 2.1.5

Bumps [onebox](https://github.com/discourse/onebox) from 2.1.4 to 2.1.5.
- [Release notes](https://github.com/discourse/onebox/releases)
- [Changelog](https://github.com/discourse/onebox/blob/master/CHANGELOG.md)
- [Commits](https://github.com/discourse/onebox/compare/v2.1.4...v2.1.5)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Gemfile.lock

@@ -235,7 +235,7 @@ GEM
     omniauth-twitter (1.4.0)
       omniauth-oauth (~> 1.1)
       rack
-    onebox (2.1.4)
+    onebox (2.1.5)
       addressable (~> 2.7.0)
       htmlentities (~> 4.3)
       multi_json (~> 1.11)