Bump rexml from 3.3.6 to 3.3.9

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] <support@github.com>

Author: dependabot[bot]

Gemfile.lock

@@ -67,8 +67,7 @@ GEM
       ffi (~> 1.0)
     rdoc (6.4.1.1)
       psych (>= 4.0.0)
-    rexml (3.3.6)
-      strscan
+    rexml (3.3.9)
     rouge (3.30.0)
     safe_yaml (1.0.5)
     sass (3.7.4)
@@ -79,7 +78,6 @@ GEM
     sassc (2.4.0)
       ffi (~> 1.9)
     stringio (3.1.0)
-    strscan (3.1.0)
     temple (0.8.2)
     terminal-table (2.0.0)
       unicode-display_width (~> 1.1, >= 1.1.1)