Bump rexml from 3.3.4 to 3.3.6

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: direct:production
...

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

Gemfile.lock

@@ -81,7 +81,7 @@ GEM
     redis-client (0.22.2)
       connection_pool
     regexp_parser (2.9.2)
-    rexml (3.3.4)
+    rexml (3.3.6)
       strscan
     rspec (3.13.0)
       rspec-core (~> 3.13.0)