Enable tmux Bash completion on ubuntu

ubuntu installs bash-completion without shipping a tmux completion file. Use the standalone tmux completion script instead. Here's how to fix it:
curl -fSsL https://raw.githubusercontent.com/imomaliev/tmux-bash-completion/master/completions/tmux \
  -o ~/.bash.tmux-bash-completion

grep -qxF '[ -r ~/.bash.tmux-bash-completion ] && . ~/.bash.tmux-bash-completion' ~/.bashrc \
  || echo '[ -r ~/.bash.tmux-bash-completion ] && . ~/.bash.tmux-bash-completion' >> ~/.bashrc