tmux: get focus events working again under 1.9a

This one snuck into tmux 1.9/1.9a and wasn't mentioned in the changelog:
a new `focus-events` option has been added and it defaults to "off",
breaking focus gained/lost events in Vim. This has been quite annoying
me at work of late, as I've had to do a lot of rebasing and amending,
and am constantly having to either `:e!` my buffers or run a manually
`:checktime` in Vim.

Here's the guilty commit:

  http://sourceforge.net/p/tmux/tmux-code/ci/c7a121cfc0137c907b7bfb/

Some additional context here:

  https://github.com/sjl/vitality.vim/issues/21

Signed-off-by: Wincent Colaiuta <win@wincent.com>
Update tmux version requirement

Now that we've added the `focus-events` option, versions of tmux older
than 1.9a will spam a warning to the console on startup. It's harmless,
but cosmetically offensive, so recommend using 1.9a or higher.

Signed-off-by: Wincent Colaiuta <win@wincent.com>