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

@@ -38,8 +38,7 @@ GEM
     rainbow (3.1.1)
     rake (13.2.1)
     regexp_parser (2.8.0)
-    rexml (3.3.6)
-      strscan
+    rexml (3.3.9)
     rspec (3.13.0)
       rspec-core (~> 3.13.0)
       rspec-expectations (~> 3.13.0)
@@ -67,7 +66,6 @@ GEM
       parser (>= 3.2.1.0)
     ruby-progressbar (1.13.0)
     ruby2_keywords (0.0.5)
-    strscan (3.1.0)
     unicode-display_width (2.4.2)
     vcr (6.1.0)
     webmock (3.23.1)