chore: Bump rexml from 3.2.8 to 3.3.9

Bumps [rexml](https://github.com/ruby/rexml) from 3.2.8 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@v3.2.8...v3.3.9)

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

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

Gemfile.lock

@@ -638,8 +638,7 @@ GEM
       railties (>= 5.0)
     reverse_markdown (2.1.1)
       nokogiri
-    rexml (3.2.8)
-      strscan (>= 3.0.9)
+    rexml (3.3.9)
     rmagick (5.3.0)
       pkg-config (~> 1.4)
     roo (2.9.0)
@@ -781,7 +780,6 @@ GEM
       net-scp (>= 1.1.2)
       net-ssh (>= 2.8.0)
     stackprof (0.2.22)
-    strscan (3.1.0)
     swd (1.3.0)
       activesupport (>= 3)
       attr_required (>= 0.0.5)