Bump rexml from 3.3.6 to 3.3.9 (#30)

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
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

Gemfile.lock

@@ -53,8 +53,7 @@ GEM
     rake (13.1.0)
     rchardet (1.8.0)
     regexp_parser (2.9.0)
-    rexml (3.3.6)
-      strscan
+    rexml (3.3.9)
     rspec (3.12.0)
       rspec-core (~> 3.12.0)
       rspec-expectations (~> 3.12.0)
@@ -104,7 +103,6 @@ GEM
       rubocop
     ruby-progressbar (1.13.0)
     ruby2_keywords (0.0.5)
-    strscan (3.1.0)
     treetop (1.6.12)
       polyglot (~> 0.3)
     tzinfo (2.0.6)