Bump rexml from 3.3.4 to 3.3.6 (#816)

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

Gemfile.lock

@@ -469,7 +469,7 @@ GEM
     responders (3.1.1)
       actionpack (>= 5.2)
       railties (>= 5.2)
-    rexml (3.3.4)
+    rexml (3.3.6)
       strscan
     rouge (4.2.0)
     rspec-core (3.13.0)