Tmux: Update tmux 2.4 compatibility (copy-mode-vi)

The new version of tmux (2.4) again includes a breaking change
in which 'bind-key -t vi-copy' has been removed and replaced by
'bind -T copy-mode-vi'.

This commit makes the 'v' and 'p' commands (under the copy mode)
work again; the configuration is compatible with both tmux 2.3
and 2.4.

tmux/tmux.conf

@@ -1,10 +1,15 @@
 # tmux.conf
-#  for tmux 2.1+ : mainly for tmux 2.3 (highly recommended)
+#  for tmux 2.1+ : mainly for tmux 2.3 and 2.4 (highly recommended)
 #
 # @seealso
 # - http://manpages.ubuntu.com/manpages/precise/man1/tmux.1.html
 
 
+# Set the TMUX_VERSION envvar (up to minor versions)
+# https://stackoverflow.com/a/40902312/1046584
+run-shell "tmux setenv -g TMUX_VERSION $(tmux -V | cut -c 6-)"
+
+
 # 1. Key Bindings {{{
 # ===================
 
@@ -114,8 +119,18 @@ bind-key Escape copy-mode
 bind-key Enter copy-mode
 bind-key ] paste-buffer
 
-bind-key -t vi-copy 'v' begin-selection      # Begin visual-selection in copy mode.
-bind-key -t vi-copy 'y' copy-pipe "pbcopy"   # Yank the current selection in copy mode.
+# vi-mode v: Begin visual-selection in copy mode.
+# vi-mode y: Yank the current selection in copy mode.
+if-shell -b '[ "$(echo "$TMUX_VERSION <= 2.3" | bc)" = 1 ]' \
+   'bind-key -t vi-copy "v" begin-selection; \
+    bind-key -t vi-copy "y" copy-pipe "pbcopy"; \
+   '
+# see https://github.com/tmux/tmux/issues/754
+if-shell -b '[ "$(echo "$TMUX_VERSION >= 2.4" | bc)" = 1 ]' \
+   'bind -T copy-mode-vi "v" send-keys -X begin-selection; \
+    bind -T copy-mode-vi "y" send-keys -X copy-pipe "pbcopy"; \
+   '
+
 bind-key 'p' run-shell "pbpaste | tmux load-buffer - && tmux paste-buffer"
 
 # no delay when switching modes (copy mode, etc.)