Bump rexml from 3.3.6 to 3.3.9

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](ruby/rexml@v3.3.6...v3.3.9)

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

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

Gemfile.lock

@@ -14,11 +14,9 @@ GEM
       rouge
       rubyzip
       tty-logger
-    rexml (3.3.6)
-      strscan
+    rexml (3.3.9)
     rouge (4.3.0)
     rubyzip (2.3.2)
-    strscan (3.1.0)
     tty-color (0.6.0)
     tty-logger (0.6.0)
       pastel (~> 0.8)