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] <support@github.com>

Author: dependabot[bot]

Gemfile.lock

@@ -107,7 +107,7 @@ GEM
       racc (~> 1.4)
     public_suffix (4.0.7)
     racc (1.8.1)
-    rexml (3.3.8)
+    rexml (3.3.9)
     ruby-macho (2.5.1)
     securerandom (0.3.1)
     typhoeus (1.4.1)