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>

Author: dependabot[bot]

Gemfile.lock

@@ -65,10 +65,8 @@ GEM
     rdf-xsd (3.3.0)
       rdf (~> 3.3)
       rexml (~> 3.2)
-    rexml (3.3.6)
-      strscan
+    rexml (3.3.9)
     scanf (1.0.0)
-    strscan (3.1.0)
     sxp (2.0.0)
       matrix (~> 0.4)
       rdf (~> 3.3)