Compare numeric tmux version only

tmuxomatic fails with tmux 1.9a fails with

```
Traceback (most recent call last):
  File "./tmuxomatic", line 1333, in <module>
    main()
  File "./tmuxomatic", line 1209, in main
    for p, q in zip( version_listint(tmux_rep), version_listint(tmux_req) ):
  File "./tmuxomatic", line 1208, in <lambda>
    version_listint = lambda version_string: [int(x) for x in version_string.split(".")]
  File "./tmuxomatic", line 1208, in <listcomp>
    version_listint = lambda version_string: [int(x) for x in version_string.split(".")]
ValueError: invalid literal for int() with base 10: '9a'
```

This uses a simple numeric only comparison on the tmux version.