improve (completion): simplify gulp completion using built in

completion/available/gulp.completion.bash

@@ -1,50 +1,5 @@
-#!/bin/bash
-# Borrowed from grunt-cli
-# http://gruntjs.com/
-#
-# Copyright jQuery Foundation and other contributors, https://jquery.org/
+# shellcheck shell=bash
 
-# This software consists of voluntary contributions made by many
-# individuals. For exact contribution history, see the revision history
-# available at https://github.com/gruntjs/grunt .
-
-# The following license applies to all parts of this software except as
-# documented below:
-
-# ====
-
-# Permission is hereby granted, free of charge, to any person obtaining
-# a copy of this software and associated documentation files (the
-# "Software"), to deal in the Software without restriction, including
-# without limitation the rights to use, copy, modify, merge, publish,
-# distribute, sublicense, and/or sell copies of the Software, and to
-# permit persons to whom the Software is furnished to do so, subject to
-# the following conditions:
-
-# The above copyright notice and this permission notice shall be
-# included in all copies or substantial portions of the Software.
-
-# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
-# EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
-# MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
-# NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
-# LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
-# OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
-# WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
-
-# Usage:
-#
-# To enable bash <tab> completion for gulp, add the following line (minus the
-# leading #, which is the bash comment character) to your ~/.bashrc file:
-#
-# eval "$(gulp --completion=bash)"
-# Enable bash autocompletion.
-function _gulp_completions() {
-# The currently-being-completed word.
-local cur="${COMP_WORDS[COMP_CWORD]}"
-#Grab tasks
-local compls=$(gulp --tasks-simple)
-# Tell complete what stuff to show.
-COMPREPLY=($(compgen -W "$compls" -- "$cur"))
-}
-complete -o default -F _gulp_completions gulp
+if _command_exists gulp; then
+	eval "$(gulp --completion=bash)"
+fi