Bump rexml from 3.3.6 to 3.3.9 (#825)

Bumps [rexml](https://github.com/ruby/rexml) from 3.3.6 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@v3.3.6...v3.3.9)

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

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

Diff for: Gemfile.lock

@@ -100,16 +100,14 @@ GEM
     open4 (1.3.4)
     public_suffix (4.0.7)
     redcarpet (3.6.0)
-    rexml (3.3.6)
-      strscan
+    rexml (3.3.9)
     rouge (4.2.0)
     ruby-macho (2.5.1)
     ruby2_keywords (0.0.5)
     sassc (2.4.0)
       ffi (~> 1.9)
     sqlite3 (1.7.2)
       mini_portile2 (~> 2.8.0)
-    strscan (3.1.0)
     typhoeus (1.4.1)
       ethon (>= 0.9.0)
     tzinfo (2.0.6)