[fish] Use $version instead of $FISH_VERSION (#1100)

$FISH_VERSION is dropped in 2.7, but every version has $version

- https://github.com/fish-shell/fish-shell/issues/4414
- https://github.com/fish-shell/fish-shell/commit/fb8ae04f80c3a129f789e7b718464d014508315f

Comment from @faho in #1316:

Unfortunately, $FISH_VERSION was only ever a thing from fish 2.0 to fish 2.7.1.

All fish versions from the very beginning though used a variable called simply "$version" to store their version, so that is the one that should be used.

shell/key-bindings.fish

@@ -40,8 +40,8 @@ function fzf_key_bindings
     begin
       set -lx FZF_DEFAULT_OPTS "--height $FZF_TMUX_HEIGHT $FZF_DEFAULT_OPTS --tiebreak=index --bind=ctrl-r:toggle-sort $FZF_CTRL_R_OPTS +m"
 
-      set -l FISH_MAJOR (echo $FISH_VERSION | cut -f1 -d.)
-      set -l FISH_MINOR (echo $FISH_VERSION | cut -f2 -d.)
+      set -l FISH_MAJOR (echo $version | cut -f1 -d.)
+      set -l FISH_MINOR (echo $version | cut -f2 -d.)
 
       # history's -z flag is needed for multi-line support.
       # history's -z flag was added in fish 2.4.0, so don't use it for versions