test dependencies

With this commit, Travis simply ensures that the installer dependencies do resolve properly. I will add "real" tests for the commands later on (you can merge this before and I can create a separate pull request later on).
Instead of `hhvm` we could use the same strategy used for `symfony`:

```yaml
php:
    # ...
    - hhvm-nightly

matrix:
    allow_failures:
        - php: hhvm-nightly
```