build(deps-dev): bump rexml from 3.3.6 to 3.3.9 (#409)

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

Gemfile.lock

@@ -220,8 +220,7 @@ GEM
       rspec-mocks (>= 3.0.0)
     resque_unit (0.4.8)
       json (>= 1.4.6)
-    rexml (3.3.6)
-      strscan
+    rexml (3.3.9)
     rspec (3.7.0)
       rspec-core (~> 3.7.0)
       rspec-expectations (~> 3.7.0)
@@ -267,7 +266,6 @@ GEM
     sshkit (1.15.1)
       net-scp (>= 1.1.2)
       net-ssh (>= 2.8.0)
-    strscan (3.1.0)
     test-unit (3.5.7)
       power_assert
     thor (1.2.1)