Update 'gulp' and use 'sass' package instead of deprecated 'node-sass'

gulpfile.js

@@ -1,5 +1,5 @@
 const _gulp = require('gulp');
-const _sass = require('gulp-sass');
+const _sass = require('gulp-sass')(require('sass'));
 const _prettyError = require('gulp-prettyerror');
 const _cleanCSS = require('gulp-clean-css');
 const _concat = require('gulp-concat');

package.json

@@ -50,9 +50,10 @@
         "gulp-clean-css": "^4.3.0",
         "gulp-concat": "^2.6.1",
         "gulp-prettyerror": "^2.0.0",
-        "gulp-sass": "^4.1.0",
+        "gulp-sass": "^5.1.0",
         "koa": "^2.13.0",
-        "less": "^3.11.3"
+        "less": "^3.11.3",
+        "sass": "^1.60.0"
     },
     "dependencies": {
         "ejs": "^3.1.3",