transport: use the new Stackdriver import path

Change-Id: I19bc4ea0cd6b01d587b379e472574da2d788b9a2
Reviewed-on: https://code-review.googlesource.com/31330
Reviewed-by: Jean de Klerk <deklerk@google.com>
diff --git a/transport/http/go18.go b/transport/http/go18.go
index 1d4bb8e..0b01c70 100644
--- a/transport/http/go18.go
+++ b/transport/http/go18.go
@@ -19,7 +19,7 @@
 import (
 	"net/http"
 
-	"go.opencensus.io/exporter/stackdriver/propagation"
+	"contrib.go.opencensus.io/exporter/stackdriver/propagation"
 	"go.opencensus.io/plugin/ochttp"
 )