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] <[email protected]>

Gemfile.lock

@@ -72,8 +72,7 @@ GEM
       racc (~> 1.4)
     racc (1.6.2)
     rake (12.3.3)
-    rexml (3.3.6)
-      strscan
+    rexml (3.3.9)
     roo (2.7.1)
       nokogiri (~> 1)
       rubyzip (~> 1.1, < 2.0.0)
@@ -95,7 +94,6 @@ GEM
     ruby-graphviz (1.2.5)
       rexml
     rubyzip (1.3.0)
-    strscan (3.1.0)
     terminal-table (3.0.2)
       unicode-display_width (>= 1.1.1, < 3)
     thread_safe (0.3.6)