Bump rexml from 3.2.5 to 3.3.9 in /ruby

Bumps [rexml](https://github.com/ruby/rexml) from 3.2.5 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@v3.2.5...v3.3.9)

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

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

ruby/Gemfile.lock

@@ -46,7 +46,7 @@ GEM
       pdf-reader (~> 2.0)
       prawn (~> 2.2)
     public_suffix (4.0.6)
-    rexml (3.2.5)
+    rexml (3.3.9)
     ruby-rc4 (0.1.5)
     safe_yaml (1.0.5)
     treetop (1.6.11)