Bump rexml from 3.3.1 to 3.3.9

Bumps [rexml](https://github.com/ruby/rexml) from 3.3.1 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@v3.3.1...v3.3.9)

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

Signed-off-by: dependabot[bot] <[email protected]>

Gemfile.lock

@@ -119,8 +119,7 @@ GEM
     rb-fsevent (0.11.2)
     rb-inotify (0.11.1)
       ffi (~> 1.0)
-    rexml (3.3.1)
-      strscan
+    rexml (3.3.9)
     rspec (3.13.0)
       rspec-core (~> 3.13.0)
       rspec-expectations (~> 3.13.0)
@@ -144,7 +143,6 @@ GEM
       nori (~> 2.4)
       wasabi (>= 3.7, < 6)
     shellany (0.0.1)
-    strscan (3.1.0)
     thor (1.3.1)
     timeout (0.4.1)
     tzinfo (2.0.6)