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>

Gemfile.lock

@@ -114,14 +114,12 @@ GEM
     open4 (1.3.4)
     public_suffix (4.0.6)
     rchardet (1.8.0)
-    rexml (3.3.6)
-      strscan
+    rexml (3.3.9)
     ruby-macho (1.4.0)
     ruby2_keywords (0.0.4)
     sawyer (0.8.2)
       addressable (>= 2.3.5)
       faraday (> 0.8, < 2.0)
-    strscan (3.1.0)
     terminal-table (3.0.0)
       unicode-display_width (~> 1.1, >= 1.1.1)
     thread_safe (0.3.6)