Update default gems list at 957473d [ci skip]

Author: matzbot

NEWS.md

@@ -65,7 +65,7 @@ The following default gems are updated.
 * json 2.11.3
 * optparse 0.7.0.dev.2
 * prism 1.4.0
-* psych 5.2.5
+* psych 5.2.6
 * stringio 3.1.8.dev
 * strscan 3.1.5.dev
 * uri 1.0.3