Disable travis emails in case of success.

Change-Id: Ie82d3b9e502e80c91cfe2f4ddc547d27d9d71a9a
diff --git a/.travis.yml b/.travis.yml
index b040cc1..c956d13 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -12,6 +12,11 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+notifications:
+  email:
+    on_success: never
+    on_failure: always
+
 language: cpp
 
 # The |CFG| variable name is hooked up by the perfetto-ci.appspot.com frontend.