Update default gems list at 9e7e938 [ci skip]

Author: matzbot

NEWS.md

@@ -198,7 +198,7 @@ The following default gems are updated.
 * prism 1.0.0
 * pstore 0.1.4
 * psych 5.2.1
-* rdoc 6.9.0
+* rdoc 6.9.1
 * reline 0.5.12
 * resolv 0.6.0
 * securerandom 0.4.1