Bump rexml from 3.3.5 to 3.3.6 in /rails

Bumps [rexml](https://github.com/ruby/rexml) from 3.3.5 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@v3.3.5...v3.3.6)

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

Signed-off-by: dependabot[bot] <support@github.com>

rails/Gemfile.lock

@@ -261,7 +261,7 @@ GEM
     responders (3.1.1)
       actionpack (>= 5.2)
       railties (>= 5.2)
-    rexml (3.3.5)
+    rexml (3.3.6)
       strscan
     rspec-core (3.13.0)
       rspec-support (~> 3.13.0)