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] <[email protected]>

Gemfile.lock

@@ -69,8 +69,7 @@ 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)
     rqrcode (2.2.0)
       chunky_png (~> 1.0)
       rqrcode_core (~> 1.0)
@@ -94,7 +93,6 @@ GEM
       rake (>= 0.8.1)
     ruby-progressbar (1.13.0)
     rumoji (0.5.0)
-    strscan (3.1.0)
     unicode-display_width (2.5.0)
     websocket (1.2.10)
     websocket-client-simple (0.8.0)