Fix: Gevent pool spawning

The latest version of gevent has removed the spawn_raw method
from the green pool object. Instead, the normal spawn is used
now which should be backwards compatible.

The spawn_raw was originally used as an optimization based on the
docs provided by gevent.

Signed-off-by: Kevin Conway <kevinjacobconway@gmail.com>
Merge pull request #11 from kevinconway/fix_gevent_spawning

Fix: Gevent pool spawning