Update rexml to version 3.3.9

Author: depfu[bot]

Gemfile.lock

@@ -56,7 +56,7 @@ GEM
     regexp_parser (2.9.2)
     require_bench (1.0.4)
       version_gem (>= 1.1.3, < 4)
-    rexml (3.3.7)
+    rexml (3.3.9)
     rots (1.0.0)
       date
       openssl