Merge pull request #5865 from dearblue/errno-to-stdlib-io

Move `mruby-errno` to `stdlib-io.gembox`

mrbgems/stdlib-io.gembox

@@ -9,4 +9,7 @@ MRuby::GemBox.new do |conf|
 
   # Use standard print/puts/p
   conf.gem :core => "mruby-print"
+
+  # Use errno extension for a good mruby-io/mruby-socket experience
+  conf.gem :core => "mruby-errno"
 end

mrbgems/stdlib.gembox

@@ -54,7 +54,4 @@ MRuby::GemBox.new do |conf|
 
   # Use class/module extension
   conf.gem :core => "mruby-class-ext"
-
-  # Use errno extension
-  conf.gem :core => "mruby-errno"
 end