Bump rexml from 3.3.8 to 3.3.9

Bumps [rexml](https://github.com/ruby/rexml) from 3.3.8 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@v3.3.8...v3.3.9)

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

Signed-off-by: dependabot[bot] <[email protected]>

Gemfile.lock

@@ -631,7 +631,7 @@ GEM
       http-cookie (>= 1.0.2, < 2.0)
       mime-types (>= 1.16, < 4.0)
       netrc (~> 0.8)
-    rexml (3.3.8)
+    rexml (3.3.9)
     rinku (2.0.6)
     rouge (4.4.0)
     rspec-core (3.13.1)