conf: upgraded rexml to fix CVE

Author: paul-mesnilgrente

Gemfile.lock

@@ -45,7 +45,7 @@ GEM
       http-cookie (>= 1.0.2, < 2.0)
       mime-types (>= 1.16, < 4.0)
       netrc (~> 0.8)
-    rexml (3.2.5)
+    rexml (3.3.8)
     rspec (3.11.0)
       rspec-core (~> 3.11.0)
       rspec-expectations (~> 3.11.0)