Bump rexml from 3.3.4 to 3.3.6 (#464)

Bumps [rexml](https://github.com/ruby/rexml) from 3.3.4 to 3.3.6.
- [Release notes](https://github.com/ruby/rexml/releases)
- [Changelog](https://github.com/ruby/rexml/blob/master/NEWS.md)
- [Commits](ruby/rexml@v3.3.4...v3.3.6)

---
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

@@ -323,7 +323,7 @@ GEM
       actionpack (>= 5.2)
       railties (>= 5.2)
     retryable (3.0.5)
-    rexml (3.3.4)
+    rexml (3.3.6)
       strscan
     rspec-core (3.12.2)
       rspec-support (~> 3.12.0)