Change gyp target types 'settings' -> 'none' as the former got removed.

This is due to http://code.google.com/p/gyp/source/detail?r=1176 and
http://code.google.com/p/chromium/issues/detail?id=96629 --- newer
versions of gyp drop the target type 'settings', as we apparently should
have been using 'none' all along.
diff --git a/icu.gyp b/icu.gyp
index 16b300e..c34c5bf 100644
--- a/icu.gyp
+++ b/icu.gyp
@@ -293,7 +293,7 @@
       'targets': [
         {
           'target_name': 'system_icu',
-          'type': 'settings',
+          'type': 'none',
           'direct_dependent_settings': {
             'defines': [
               'USE_SYSTEM_ICU',