Bump rexml from 3.3.3 to 3.3.6

Bumps [rexml](https://github.com/ruby/rexml) from 3.3.3 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@v3.3.3...v3.3.6)

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

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

Gemfile.lock

@@ -180,7 +180,7 @@ GEM
     redcarpet (3.6.0)
     request_store (1.5.1)
       rack (>= 1.4)
-    rexml (3.3.3)
+    rexml (3.3.6)
       strscan
     sass (3.7.4)
       sass-listen (~> 4.0.0)