The default confusingly seems to change depending on the notification channel. 

http://docs.travis-ci.com/user/notifications/

Under Email:
```
on_success: [always|never|change] # default: change
on_failure: [always|never|change] # default: always
```

Under IRC:
```
on_success: [always|never|change] # default: always
on_failure: [always|never|change] # default: always
```

Under Webhook:
```
on_success: [always|never|change] # default: always
on_failure: [always|never|change] # default: always
on_start: [true|false] # default: false
```