Update `host*` targets to use `default.gembox`.

And now `default.gembox` includes `mruby-socket` gem.

mrbgems/default.gembox

@@ -5,6 +5,9 @@ MRuby::GemBox.new do |conf|
   # Use standard IO/File class
   conf.gem :core => "mruby-io"
 
+  # Use standard IO/File class
+  conf.gem :core => "mruby-socket"
+
   # Use standard Array#pack, String#unpack methods
   conf.gem :core => "mruby-pack"
 

target/host-debug.rb

@@ -11,7 +11,7 @@ MRuby::Build.new('host-debug') do |conf|
   enable_debug
 
   # include the default GEMs
-  conf.gembox 'full-core'
+  conf.gembox 'default'
 
   # C compiler settings
   conf.cc.defines = %w(MRB_ENABLE_DEBUG_HOOK)

target/host-shared.rb

@@ -9,7 +9,7 @@ MRuby::Build.new do |conf|
   end
 
   # include the GEM box
-  conf.gembox 'full-core'
+  conf.gembox 'default'
 
   # C compiler settings
   conf.cc do |cc|

target/host.rb

@@ -20,7 +20,7 @@ MRuby::Build.new do |conf|
   # conf.gem :git => 'git@github.com:mattn/mruby-onig-regexp.git', :branch => 'master', :options => '-v'
 
   # include the GEM box
-  conf.gembox 'full-core'
+  conf.gembox 'default'
 
   # C compiler settings
   # conf.cc do |cc|