Update default gems list at 67869b2 [ci skip]

Author: matzbot

NEWS.md

@@ -188,7 +188,7 @@ The following default gems are updated.
 * io-console 0.8.1
 * io-nonblock 0.3.2
 * io-wait 0.3.2
-* json 2.14.1
+* json 2.15.0
 * optparse 0.7.0.dev.2
 * prism 1.5.1
 * psych 5.2.6