fix .travis.yml

before

```
% travis lint .travis.yml
Warnings for .travis.yml:
[x] value for notifications section is empty, dropping
[x] in notifications section: unexpected key on_success, dropping
[x] in notifications section: unexpected key on_failure, dropping
```

now:

```
Hooray, .travis.yml looks valid :)
```

See also ProgVal/Supybot-plugins#217