Update gulp

package.json

@@ -47,13 +47,13 @@
         "beeper": "1.1.1",
         "cssnano": "4.1.10",
         "gscan": "2.2.1",
-        "gulp": "3.9.1",
+        "gulp": "4.0.0",
         "gulp-filter": "5.1.0",
-        "gulp-livereload": "3.8.1",
-        "gulp-postcss": "6.4.0",
-        "gulp-sourcemaps": "1.12.1",
+        "gulp-livereload": "4.0.1",
+        "gulp-postcss": "8.0.0",
+        "gulp-sourcemaps": "2.6.5",
         "gulp-uglify": "3.0.2",
-        "gulp-watch": "4.3.11",
+        "gulp-watch": "5.0.1",
         "gulp-zip": "4.2.0",
         "postcss-color-function": "4.0.1",
         "postcss-custom-properties": "8.0.9",