Bump rexml from 3.2.8 to 3.3.9 (#46)

Bumps [rexml](https://github.com/ruby/rexml) from 3.2.8 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@v3.2.8...v3.3.9)

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

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

Gemfile.lock

@@ -77,8 +77,7 @@ GEM
     regexp_parser (2.2.1)
     reverse_markdown (1.3.0)
       nokogiri
-    rexml (3.2.8)
-      strscan (>= 3.0.9)
+    rexml (3.3.9)
     rspec (3.9.0)
       rspec-core (~> 3.9.0)
       rspec-expectations (~> 3.9.0)
@@ -126,7 +125,6 @@ GEM
       thor (~> 0.19, >= 0.19.4)
       tilt (~> 2.0)
       yard (~> 0.9)
-    strscan (3.1.0)
     thor (0.20.3)
     tilt (2.0.10)
     timecop (0.9.5)