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

@@ -19,8 +19,7 @@ GEM
     hashdiff (1.0.1)
     public_suffix (5.0.3)
     rake (13.0.6)
-    rexml (3.3.6)
-      strscan
+    rexml (3.3.9)
     rspec (3.12.0)
       rspec-core (~> 3.12.0)
       rspec-expectations (~> 3.12.0)
@@ -35,7 +34,6 @@ GEM
       rspec-support (~> 3.12.0)
     rspec-support (3.12.1)
     ruby2_keywords (0.0.5)
-    strscan (3.1.0)
     webmock (3.18.1)
       addressable (>= 2.8.0)
       crack (>= 0.3.2)