Bump rexml from 3.3.6 to 3.3.9 (#430)

Bumps [rexml](https://github.com/ruby/rexml) from 3.3.6 to 3.3.9.
- [Release notes](https://github.com/ruby/rexml/releases)
- [Changelog](https://github.com/ruby/rexml/blob/master/NEWS.md)
- [Commits](https://github.com/ruby/rexml/compare/v3.3.6...v3.3.9)

---
updated-dependencies:
- dependency-name: rexml
  dependency-type: indirect
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Riccardo Graziosi <31478034+riggraz@users.noreply.github.com>

Gemfile.lock

@@ -222,7 +222,7 @@ GEM
     responders (3.0.1)
       actionpack (>= 5.0)
       railties (>= 5.0)
-    rexml (3.3.8)
+    rexml (3.3.9)
     rspec-core (3.12.2)
       rspec-support (~> 3.12.0)
     rspec-expectations (3.12.3)