Fix for pymongo > 3.0 and connection support of MongoClient only

https://api.mongodb.com/python/current/changelog.html#mongoclient-changes

plugins/mongodb/mongo_lag

@@ -25,7 +25,7 @@ import pymongo
 def _get_members():
     host = os.environ.get('host', '127.0.0.1')
     port = os.environ.get('port', 27017)
-    conn = pymongo.Connection(host,port)
+    conn = pymongo.MongoClient(host,port)
     repl_status = conn.admin.command("replSetGetStatus")
 
     members = {}