Bump rexml from 3.2.5 to 3.3.9

Bumps [rexml](https://github.com/ruby/rexml) from 3.2.5 to 3.3.9.
- [Release notes](https://github.com/ruby/rexml/releases)
- [Changelog](https://github.com/ruby/rexml/blob/master/NEWS.md)
- [Commits](ruby/rexml@v3.2.5...v3.3.9)

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

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

Gemfile.lock

@@ -35,8 +35,8 @@ GEM
       jekyll (>= 3.3, < 5.0)
     jekyll-watch (2.2.1)
       listen (~> 3.0)
-    kramdown (2.3.1)
-      rexml (~> 3.2.5)
+    kramdown (2.5.1)
+      rexml (>= 3.3.9)
     kramdown-parser-gfm (1.1.0)
       kramdown (~> 2.0)
     liquid (4.0.3)
@@ -50,7 +50,7 @@ GEM
     rb-fsevent (0.10.4)
     rb-inotify (0.10.1)
       ffi (~> 1.0)
-    rexml (3.2.5)
+    rexml (3.3.9)
     rouge (3.23.0)
     safe_yaml (1.0.5)
     sassc (2.4.0)