Update dependency rexml to v3.3.6 [SECURITY]

Author: renovate[bot]

Gemfile.lock

@@ -251,7 +251,7 @@ GEM
     responders (3.0.1)
       actionpack (>= 5.0)
       railties (>= 5.0)
-    rexml (3.3.4)
+    rexml (3.3.6)
       strscan
     rspec-core (3.13.0)
       rspec-support (~> 3.13.0)