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/zlib.gyp b/zlib.gyp
index 52db72a..b6bc7c5 100644
--- a/zlib.gyp
+++ b/zlib.gyp
@@ -60,7 +60,7 @@
       'targets': [
         {
           'target_name': 'zlib',
-          'type': 'settings',
+          'type': 'none',
           'direct_dependent_settings': {
             'defines': [
               'USE_SYSTEM_ZLIB',