Bump rexml from 3.3.4 to 3.3.9

Bumps [rexml](https://github.com/ruby/rexml) from 3.3.4 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@v3.3.4...v3.3.9)

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

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

Gemfile.lock

@@ -54,8 +54,7 @@ GEM
     os (1.1.4)
     racc (1.8.1)
     rake (13.0.6)
-    rexml (3.3.4)
-      strscan
+    rexml (3.3.9)
     ronn-ng (0.10.1)
       kramdown (~> 2, >= 2.1)
       kramdown-parser-gfm (~> 1, >= 1.0.1)
@@ -74,7 +73,6 @@ GEM
       diff-lcs (>= 1.2.0, < 2.0)
       rspec-support (~> 3.12.0)
     rspec-support (3.12.2)
-    strscan (3.1.0)
     sys-uname (1.3.0)
       ffi (~> 1.1)
     thor (1.3.1)