Bump rexml from 3.3.8 to 3.3.9 in /asf-site-src (#337)

Author: dependabot[bot]

asf-site-src/Gemfile.lock

@@ -108,7 +108,7 @@ GEM
     rb-fsevent (0.11.2)
     rb-inotify (0.11.1)
       ffi (~> 1.0)
-    rexml (3.3.8)
+    rexml (3.3.9)
     rouge (3.30.0)
     sassc (2.4.0)
       ffi (~> 1.9)