Update rdoc to 6.13.1

Author: ksss

Gemfile.lock

@@ -96,7 +96,7 @@ GEM
     rb-fsevent (0.11.2)
     rb-inotify (0.11.1)
       ffi (~> 1.0)
-    rdoc (6.13.0)
+    rdoc (6.13.1)
       psych (>= 4.0.0)
     regexp_parser (2.10.0)
     rspec (3.13.0)