Refactor Window#build_panes to always return an Array

This is because it is treated like an Array in `Window#panes?` and in `lib/tmuxinator/assets/template.erb`

Without this, the following YAML would fail:

```yaml
windows:
  - vim:
      pre: "some_command"
```

and so would this:

```yaml
windows:
  - vim:
      panes: "vim"
```

I also refactored the specs a little :smile: