Bump rexml from 3.3.8 to 3.4.0

Bumps [rexml](https://github.com/ruby/rexml) from 3.3.8 to 3.4.0.
- [Release notes](https://github.com/ruby/rexml/releases)
- [Changelog](https://github.com/ruby/rexml/blob/master/NEWS.md)
- [Commits](ruby/rexml@v3.3.8...v3.4.0)

---
updated-dependencies:
- dependency-name: rexml
  dependency-type: indirect
  update-type: version-update:semver-minor
...

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

Gemfile.lock

@@ -260,7 +260,7 @@ GEM
     rb-fsevent (0.11.2)
     rb-inotify (0.11.1)
       ffi (~> 1.0)
-    rexml (3.3.8)
+    rexml (3.4.0)
     rouge (3.30.0)
     rspec (3.13.0)
       rspec-core (~> 3.13.0)