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

@@ -150,8 +150,7 @@ GEM
       ffi (~> 1.0)
     redcarpet (3.5.1)
     regexp_parser (2.6.1)
-    rexml (3.3.6)
-      strscan
+    rexml (3.3.9)
     rouge (3.30.0)
     rspec (3.12.0)
       rspec-core (~> 3.12.0)
@@ -174,7 +173,6 @@ GEM
     sassc (2.4.0)
       ffi (~> 1.9)
     servolux (0.13.0)
-    strscan (3.1.0)
     temple (0.9.1)
     terser (1.1.13)
       execjs (>= 0.3.0, < 3)