frontend: migrate to gulp 4

frontend/package.json

@@ -9,7 +9,7 @@
   "devDependencies": {
     "chartist": "0.11.0",
     "fixmyjs": "2.0.0",
-    "gulp": "3.9.1",
+    "gulp": "4.0.2",
     "gulp-clean-css": "3.9.4",
     "gulp-concat": "2.6.1",
     "gulp-eslint": "5.0.0",
@@ -21,11 +21,11 @@
     "highlightjs": "9.10.0",
     "html-minifier": "3.5.7",
     "jquery": "3.2.1",
-    "natives": "^1.1.6",
     "normalize-scss": "7.0.1",
     "sass": "1.5.1",
     "uglify-js": "3.4.1",
-    "vue": "2.5.16"
+    "vue": "2.5.16",
+    "node-sass": "4.12.0"
   },
   "dependencies": {
     "eslint": "^5.10.0"