Fix running commands in the panes

Hello,

after upgrading to 0.6.5  I encounter error, when I tried to run command in multi pane windows, they didn't get ran at all with tmux warning: ```can't find pane: n``` where ```n``` was the number of the pane.

After some debugging I found that command is called before pane was created. My env:

    tmuxinator 0.6.5
    tmux 1.8.0 HEAD
    zsh 5.0.2

My testing configuration:
    # ~/.tmuxinator/sct.yml

    project_name: Client
    project_root: ~/Projects/fun/client.com
    pre_windows: 1.9.3@client.com
    windows:
      - git:
      - editor: vim
      - shell:
          layout: main-vertical
          panes:
            - bundle exec padrino start -p 3001
            - bundle exec padrino console
            -
      - test: rspec && cucumber

This patch fixes the problem for me, but I have not been able to solve how I should add regression test for it. Any directions would be appreciated.

Hope it all makes sense and thank you for this timesaver.