Update default gems list at 502fcfc [ci skip]

Author: matzbot

NEWS.md

@@ -57,7 +57,7 @@ The following default gems are updated.
 * optparse 0.7.0.dev.2
 * prism 1.4.0
 * psych 5.2.3
-* stringio 3.1.6.dev
+* stringio 3.1.6
 * uri 1.0.3
 
 The following bundled gems are added.