build: send build fail/pass notifications to mailing list

Bring this repo in line with the main repo.

Change-Id: I355f7c40603b832cf2145144ff68ed6146636e51
Reviewed-on: https://upspin-review.googlesource.com/13120
Reviewed-by: Rob Pike <r@golang.org>
diff --git a/.travis.yml b/.travis.yml
index 3745bdb..6b8dc5b 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -3,3 +3,6 @@
 - 1.8.x
 go_import_path: exp.upspin.io
 script: go test -short -race ./...
+notifications:
+  email:
+  - upspin-reviews@googlegroups.com