Fix tests with latest pymongo/mongoengine.

mongoengine>=0.9.0 and pymongo>=2.1 require extra parameters:

- The server connection timeout was set too high
- We have to define a ``read_preference``.

tests/test_mongoengine.py

@@ -61,10 +61,20 @@ class MongoEngineTestCase(unittest.TestCase):
     db_name = os.environ.get('MONGO_DATABASE', 'factory_boy_test')
     db_host = os.environ.get('MONGO_HOST', 'localhost')
     db_port = int(os.environ.get('MONGO_PORT', '27017'))
+    MONGOD_TIMEOUT_MS = 100
 
     @classmethod
     def setUpClass(cls):
-        cls.db = mongoengine.connect(cls.db_name, host=cls.db_host, port=cls.db_port)
+        from pymongo import read_preferences as mongo_rp
+        cls.db = mongoengine.connect(
+            db=cls.db_name,
+            host=cls.db_host,
+            port=cls.db_port,
+            # PyMongo>=2.1 requires an explicit read_preference.
+            read_preference=mongo_rp.ReadPreference.PRIMARY,
+            # PyMongo>=2.1 has a 20s timeout, use 100ms instead
+            serverselectiontimeoutms=cls.MONGOD_TIMEOUT_MS,
+        )
 
     @classmethod
     def tearDownClass(cls):