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](https://github.com/ruby/rexml/compare/v3.3.6...v3.3.9)

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

Signed-off-by: dependabot[bot] <support@github.com>

Gemfile.lock

@@ -257,8 +257,7 @@ GEM
       http-cookie (>= 1.0.2, < 2.0)
       mime-types (>= 1.16, < 4.0)
       netrc (~> 0.8)
-    rexml (3.3.6)
-      strscan
+    rexml (3.3.9)
     rinku (2.0.6)
     rszr (1.3.0)
     ruby-progressbar (1.13.0)
@@ -311,7 +310,6 @@ GEM
       dante (>= 0.2.0)
       multi_json (~> 1.0)
       stripe (> 5, < 6)
-    strscan (3.1.0)
     sync (0.5.0)
     term-ansicolor (1.7.1)
       tins (~> 1.0)