Fix tmux <= 1.6 quiet option.

Sections of guard pass -q to tmux, which is an option added in tmux 1.7.
This option is now inserted by the _quiet_option function, which is
dependent upon the tmux version.

Related to #512
Merge pull request #586 from gmacdougall/fix-tmux-quiet-option

Fix tmux <= 1.6 quiet option.