service account is needed to create pod in 2.6.3 (#564)

This cause gitlab trigger failed when create pod

examples/sensors/gitlab.yaml

@@ -43,6 +43,7 @@ spec:
                     value: hello world
                 templates:
                 - name: whalesay
+                  serviceAccountName: argo-events-sa
                   inputs:
                     parameters:
                     - name: message