Tmux supported version needs to be updated

[mchouque]

Hello,

I'm dealing with multiple environments and multiple OS versions and in the README, the required version for tmux is incorrect.

tmux -V >= 2.1

tmux-powerline (in tag v3.0.0) uses a tmux variable called status-format which got introduced in tmux 2.9.

So the requirement for v3.0.0 is at least >= 2.9 (I haven't checked everything but I checked that v2.1.0 works with tmux 2.7).

Regards, Mathieu