Fix travis build config warning

```
Build config validation 
root: deprecated key sudo (The key `sudo` has no effect anymore.)
root: missing os, using the default linux
```

## TODO:
- remove `sudo`
  - https://blog.travis-ci.com/2018-11-19-required-linux-infrastructure-migration
- explicitly define os
  - https://docs.travis-ci.com/user/multi-os/