Update gulp to v4

.gulpfile.js

 /*
- * This file is part of ARSnova Cards.
- * Copyright (C) 2016 The ARSnova Team
+ * This file is part of cards.
+ * Copyright (C) 2018 The ARSnova Team
  *
- * ARSnova Click is free software: you can redistribute it and/or modify
+ * cards is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License as published by
  * the Free Software Foundation, either version 3 of the License, or
  * (at your option) any later version.
  *
- * ARSnova Click is distributed in the hope that it will be useful,
+ * cards is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  * GNU General Public License for more details.
  *
  * You should have received a copy of the GNU General Public License
- * along with ARSnova Click.  If not, see <http://www.gnu.org/licenses/>.*/
+ * along with cards.  If not, see <http://www.gnu.org/licenses/>.*/
 
-var gulp = require('gulp'),
+let gulp = require('gulp4'),
 	watch = require('gulp-watch');
-var jshint = require('gulp-jshint');
-var jscs = require('gulp-jscs');
-var stylish = require('jshint-stylish');
+let jshint = require('gulp-jshint');
+let jscs = require('gulp-jscs');
+let stylish = require('jshint-stylish');
 
-var paths = [
+let paths = [
 	'./{client,imports,i18n,server,tests}/**/*.js'
 ];
 
-gulp.task('default', ['codeCheck']);
-
 gulp.task('watch', function () {
-	gulp.start('codeCheck');
-	gulp.watch(paths, ['codeCheck'])
-});
-
-gulp.task('codeCheck', function () {
-	gulp.start('lint');
-	gulp.start('jscs');
+	gulp.watch(paths, gulp.series('codeCheck'));
 });
 
 gulp.task('lint', function () {
@@ -48,3 +40,13 @@ gulp.task('jscs', function () {
 		.pipe(jscs())
 		.pipe(jscs.reporter());
 });
+
+gulp.task(
+	'codeCheck',
+	gulp.series('lint', 'jscs')
+);
+
+gulp.task(
+	'default',
+	gulp.series('codeCheck')
+);

package.json

@@ -21,13 +21,13 @@
   "version": "1.0.0",
   "main": "index.js",
   "devDependencies": {
-    "gulp": "^3.9.1",
+    "chimp": "^0.50.2",
     "gulp-jscs": "^3.0.2",
     "gulp-jshint": "^2.0.0",
     "gulp-watch": "^4.3.5",
+    "gulp4": "^4.0.2",
     "jshint": "^2.9.5",
-    "jshint-stylish": "^2.1.0",
-    "chimp": "^0.50.2"
+    "jshint-stylish": "^2.1.0"
   },
   "author": "",
   "license": "ISC",