Completely disable Travis CI emails

They IMO are mostly a source of noise.

Change-Id: I4877d468fd060ca66e1d451397239a18fd5963d4
diff --git a/.travis.yml b/.travis.yml
index 3013aaa..315cdfc 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -13,9 +13,7 @@
 # limitations under the License.
 
 notifications:
-  email:
-    on_success: never
-    on_failure: always
+  email: false
 
 language: cpp