chore(deps-dev): 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

@@ -219,8 +219,7 @@ GEM
     responders (3.1.1)
       actionpack (>= 5.2)
       railties (>= 5.2)
-    rexml (3.3.6)
-      strscan
+    rexml (3.3.9)
     rollbar (3.5.2)
     rspec-core (3.13.1)
       rspec-support (~> 3.13.0)
@@ -286,7 +285,6 @@ GEM
       mini_portile2 (~> 2.8.0)
     sqlite3 (2.1.0-x86_64-linux-gnu)
     stringio (3.1.1)
-    strscan (3.1.0)
     thor (1.3.2)
     timeout (0.4.1)
     tzinfo (2.0.6)