tmux: use recommend cross-platform config

Enabling copy-paste inside tmux on OSX requires
``reattach-to-user-namepsace``. However, including the script in the
default ``tmux.conf`` meant that ``tmux`` wouldn't launch properly on
non-OSX systems (e.g. Ubuntu). My previous work-around (relying on an
additional in-between script to check the OS) worked, but wasn't very
elegant. So now I've switch to the recommended cross-platform config
(which checks the OS indirectly inside ``tmux.conf``).