Fix issue when no pane are specified

I don't use *panes* in my tmux configuration, each of my *windows* only have the main *pane*. This was creating an issue where `self.panes` was `nil`.

Teamocil was trying to run the `splits` method which doesn't seem to exist. I've replaced the assignment with an empty Array.

Not sure it's the correct way to fix it but it works perfectly for my situation.
This line was there to keep supporting the `splits` key (that was dropped for the `panes` key). But I realize now that this isn’t even working — the class doesn’t support the `splits` attribute.

I think we can drop the `splits` key support now. I’ll push a fix in master that includes tests.

Thanks @garno!