Bump rexml from 3.3.5 to 3.3.9

Bumps [rexml](https://github.com/ruby/rexml) from 3.3.5 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@v3.3.5...v3.3.9)

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

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

Gemfile.lock

@@ -25,8 +25,7 @@ GEM
     rdoc (6.7.0)
       psych (>= 4.0.0)
     regexp_parser (2.9.2)
-    rexml (3.3.5)
-      strscan
+    rexml (3.3.9)
     rspec (3.13.0)
       rspec-core (~> 3.13.0)
       rspec-expectations (~> 3.13.0)
@@ -69,7 +68,6 @@ GEM
     simplecov-lcov (0.8.0)
     simplecov_json_formatter (0.1.4)
     stringio (3.1.1)
-    strscan (3.1.0)
     test-unit (3.6.2)
       power_assert
     unicode-display_width (2.5.0)