Bump rexml from 3.3.3 to 3.3.6 (#630)

Author: dependabot[bot]

Gemfile.lock

@@ -7,7 +7,7 @@ GEM
       ast (~> 2.4.1)
     rainbow (3.0.0)
     regexp_parser (2.1.1)
-    rexml (3.3.3)
+    rexml (3.3.6)
       strscan
     rubocop (1.22.1)
       parallel (~> 1.10)