Generalize the tmux version logic.

This code should never have been where it was. I was lazy in reviewing #144 which did not appropriately scope the version check to tmux. This fixes #148.
Use the `-g` option for tmux 1.9 compatibility.
Replace ack.vim with ag.vim.

Fixes #141.
No need to configure ack.vim.
Update the README to reflect ack.vim -> ag.vim.
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