add an exception at setup time for old python.

setup.py

@@ -35,6 +35,8 @@ if WIN:
     if not os.environ.get('PYEXE'):
         os.environ['PYEXE'] = os.environ['PYTHON_EXE']
 
+if sys.version_info[:2] < (2, 7):
+    raise Exception("Please install gevent 1.1 for Python 2.6")
 
 import distutils
 import distutils.sysconfig  # to get CFLAGS to pass into c-ares configure script