better support for new and old gevent dns

Thanks @denik. Fixes #336.

gunicorn/workers/ggevent.py

@@ -20,7 +20,6 @@ except ImportError:
 from gevent.pool import Pool
 from gevent.server import StreamServer
 from gevent import pywsgi
-from gevent import core
 
 import gunicorn
 from gunicorn.workers.async import AsyncWorker
@@ -88,7 +87,7 @@ class GeventWorker(AsyncWorker):
         except gevent.GreenletExit:
             pass
 
-    if hasattr(gevent.core, 'dns_shutdown'):
+    if gevent.version_info[0] == 0:
 
         def init_process(self):
             #gevent 0.13 and older doesn't reinitialize dns for us after forking