Doesn't work well with HEAD version of tmux

[demon386]

When using HEAD version of tmux, I fail to load the second YAML for tmuxp.

E.g.

1. Initially, tmux has no session.
2. tmuxp load xxx.yaml -> OK
3. tmuxp load yyy.yaml ->

   Session name yyy is already running. Attach? [Y/n]

Actually this session is NOT already running.

Pressing "Y", I got

  File "/usr/local/bin/tmuxp", line 9, in <module>
    load_entry_point('tmuxp==0.9.1', 'console_scripts', 'tmuxp')()
  File "/Library/Python/2.7/site-packages/tmuxp/cli.py", line 968, in main
    command_load(args)
  File "/Library/Python/2.7/site-packages/tmuxp/cli.py", line 422, in command_load
    load_workspace(file_user, args)
  File "/Library/Python/2.7/site-packages/tmuxp/cli.py", line 295, in load_workspace
    builder.session.attach_session()
AttributeError: 'NoneType' object has no attribute 'attach_session'

[tony]

we'll switch back travis to test against tmux latest git again

[tony]

Thank you for spotting this!

Regarding your experience in particular, 0.9.2 is updated to work with tmux 2.1.

Our travis builds are testing against tmux' git master once again.