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

@@ -156,8 +156,7 @@ GEM
       rdf (~> 3.3)
       rexml (~> 3.2)
     redcarpet (3.6.0)
-    rexml (3.3.6)
-      strscan
+    rexml (3.3.9)
     scanf (1.0.0)
     shacl (0.4.1)
       json-ld (~> 3.3)
@@ -186,7 +185,6 @@ GEM
       net-http-persistent (~> 4.0, >= 4.0.2)
       rdf (~> 3.3)
     stringio (3.1.0)
-    strscan (3.1.0)
     sxp (1.3.0)
       matrix (~> 0.4)
       rdf (~> 3.3)