Add jwt.Credentials.from_signing_credentials, remove serivce_account.Credentials.to_jwt_credentials (#120)

diff --git a/system_tests/test_grpc.py b/system_tests/test_grpc.py
index 8a4419a..4bf1c5b 100644
--- a/system_tests/test_grpc.py
+++ b/system_tests/test_grpc.py
@@ -14,6 +14,7 @@
 
 import google.auth
 import google.auth.credentials
+import google.auth.jwt
 import google.auth.transport.grpc
 from google.cloud.gapic.pubsub.v1 import publisher_client
 
@@ -42,7 +43,8 @@
     credentials, project_id = google.auth.default()
     audience = 'https://{}/google.pubsub.v1.Publisher'.format(
         publisher_client.PublisherClient.SERVICE_ADDRESS)
-    credentials = credentials.to_jwt_credentials(
+    credentials = google.auth.jwt.Credentials.from_signing_credentials(
+        credentials,
         audience=audience)
 
     channel = google.auth.transport.grpc.secure_authorized_channel(