.travis.yml: Also run examples in Neovim mode.

Fixes #64.
Yes, unfortunately.. Travis will chain everything together on [one line](https://travis-ci.org/google/vroom/jobs/38897858#L30), so it is basically the same as

```
if [ $CI_TARGET = neovim ]; then pip install neovim; fi
```

I didn't find another way to create multi-line statements.