build(deps): bump rexml from 3.3.9 to 3.4.2 in /example

Bumps [rexml](https://github.com/ruby/rexml) from 3.3.9 to 3.4.2.
- [Release notes](https://github.com/ruby/rexml/releases)
- [Changelog](https://github.com/ruby/rexml/blob/master/NEWS.md)
- [Commits](ruby/rexml@v3.3.9...v3.4.2)

---
updated-dependencies:
- dependency-name: rexml
  dependency-version: 3.4.2
  dependency-type: indirect
...

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

Author: dependabot[bot]

example/Gemfile.lock

@@ -81,7 +81,7 @@ GEM
     netrc (0.11.0)
     nkf (0.2.0)
     public_suffix (4.0.7)
-    rexml (3.3.9)
+    rexml (3.4.2)
     ruby-macho (2.5.1)
     typhoeus (1.4.1)
       ethon (>= 0.9.0)