travis: use preinstalled docker to run azurite

We use it on linux only anyway and ubuntu xenial on travis has [docker preinstalled](https://docs.travis-ci.com/user/reference/xenial/#docker). That one could be [activated with](https://docs.travis-ci.com/user/docker/):
```yaml
services:
  - docker
```
We will save on `apt-get update; apt-get install ...` time.