convert gulpfile to gulp 4 version

bin/generator.js

@@ -63,8 +63,6 @@ async function generator(configFilename, pageDefinitionFile, distPath){
         // inject footer
         pconf.footer = pconf.footer || config.footer;
 
-        
-
         // render page
         const content = await _pageRenderer(tpl, css, pconf);
 

gulpfile.js

@@ -4,8 +4,6 @@ const _prettyError = require('gulp-prettyerror');
 const _cleanCSS = require('gulp-clean-css');
 const _concat = require('gulp-concat-util');
 
-_gulp.task('default', ['sass']);
- 
 _gulp.task('sass', function (){
     return _gulp.src('./assets/layout.scss')
         .pipe(_prettyError())
@@ -15,5 +13,9 @@ _gulp.task('sass', function (){
         .pipe(_concat('layout.css'))
 
         .pipe(_gulp.dest('./assets'));
-});
+}); 
+
+_gulp.task('default', _gulp.series(['sass']));
+ 
+