Prevent unnecessary Travis CI emails on each pass

Author: kaushalmodi

.travis.yml

@@ -78,4 +78,9 @@ deploy:
   on:
     tags: true
 
+notifications:
+  email:
+    on_success: never # default: change
+    on_failure: always # default: always
+
 # Ref: https://github.com/kaushalmodi/hello_musl/blob/master/.travis.yml