Update versions of dependencies and make gulp compatible with them.

dist/css/splide-core.min.css

@@ -1 +1 @@
-@keyframes splide-loading{0%{transform:rotate(0)}to{transform:rotate(1turn)}}.splide{position:relative;visibility:hidden}.splide__track{position:relative;z-index:0;overflow:hidden}.splide__list{display:flex}.splide__slide{position:relative;box-sizing:border-box}.splide__slide.is-loading{display:flex;justify-content:center;align-items:center}.splide__slide img{vertical-align:bottom}.splide__container{box-sizing:border-box}.splide__spinner{display:inline-block;width:20px;height:20px;border-radius:50%;border:2px solid #999;border-left-color:transparent;animation:splide-loading 1s infinite linear}.splide__pagination{display:inline-flex;align-items:center}.splide__pagination li{display:inline-flex}.splide__sr{border:0!important;clip:rect(1px,1px,1px,1px)!important;clip-path:inset(50%)!important;height:1px!important;overflow:hidden!important;padding:0!important;position:absolute!important;width:1px!important;white-space:nowrap!important}.splide--draggable>.splide__track>.splide__list>.splide__slide{user-select:none}.splide--fade>.splide__track>.splide__list{display:block}.splide--fade>.splide__track>.splide__list>.splide__slide{position:absolute;top:0;left:0;z-index:0;opacity:0}.splide--fade>.splide__track>.splide__list>.splide__slide.is-active{position:relative;z-index:1;opacity:1}.splide--rtl{direction:rtl}.splide--ttb>.splide__track>.splide__list{display:block}
+@keyframes splide-loading{0%{transform:rotate(0)}to{transform:rotate(1turn)}}.splide{position:relative;visibility:hidden}.splide__track{position:relative;z-index:0;overflow:hidden}.splide__list{display:flex}.splide__slide{position:relative;box-sizing:border-box}.splide__slide.is-loading{display:flex;justify-content:center;align-items:center}.splide__slide img{vertical-align:bottom}.splide__container{box-sizing:border-box}.splide__spinner{display:inline-block;width:20px;height:20px;border-radius:50%;border:2px solid #999;border-left-color:transparent;animation:splide-loading 1s linear infinite}.splide__pagination{display:inline-flex;align-items:center}.splide__pagination li{display:inline-flex}.splide__sr{border:0!important;clip:rect(1px,1px,1px,1px)!important;-webkit-clip-path:inset(50%)!important;clip-path:inset(50%)!important;height:1px!important;overflow:hidden!important;padding:0!important;position:absolute!important;width:1px!important;white-space:nowrap!important}.splide--draggable>.splide__track>.splide__list>.splide__slide{-webkit-user-select:none;user-select:none}.splide--fade>.splide__track>.splide__list{display:block}.splide--fade>.splide__track>.splide__list>.splide__slide{position:absolute;top:0;left:0;z-index:0;opacity:0}.splide--fade>.splide__track>.splide__list>.splide__slide.is-active{position:relative;z-index:1;opacity:1}.splide--rtl{direction:rtl}.splide--ttb>.splide__track>.splide__list{display:block}

gulpfile.js

@@ -48,18 +48,20 @@ const css = {
 /*
  * Build a script file.
  */
-gulp.task( 'build:js', () => {
+gulp.task( 'build:js', done => {
 	js.forEach( path => {
-		return webpackStream( { config: require( path ) } )
+		webpackStream( { config: require( path ) } )
 			.pipe( rename( { suffix: '.min' } ) )
 			.pipe( gulp.dest( './dist/js' ) );
 	} );
+
+	done();
 } );
 
 /*
  * Build sass files.
  */
-gulp.task( 'build:sass', () => {
+gulp.task( 'build:sass', done => {
 	Object.values( css ).forEach( settings => {
 		let stream = gulp.src( settings.path );
 
@@ -73,11 +75,13 @@ gulp.task( 'build:sass', () => {
 			.pipe( sass() )
 			.pipe( postcss( [
 				cssnano( { reduceIdents: false } ),
-				autoprefixer( { browsers: [ '> 5%' ] } )
+				autoprefixer( { overrideBrowserslist: [ '> 5%' ] } )
 			] ) )
 			.pipe( rename( { suffix: '.min' } ) )
 			.pipe( gulp.dest( settings.dest ) );
 	} );
+
+	done();
 } );
 
 gulp.task( 'lint', () => {

package.json

@@ -24,18 +24,18 @@
   "devDependencies": {
     "@babel/core": "^7.5.5",
     "@babel/preset-env": "^7.5.5",
-    "autoprefixer": "^8.4.1",
+    "autoprefixer": "^9.6.1",
     "babel-jest": "^24.8.0",
     "babel-loader": "^8.0.6",
-    "cssnano": "^3.10.0",
-    "gulp": "^3.9.1",
+    "cssnano": "^4.1.10",
+    "gulp": "^4.0.2",
     "gulp-concat": "^2.6.1",
     "gulp-eslint": "^6.0.0",
-    "gulp-postcss": "^7.0.1",
-    "gulp-rename": "^1.2.2",
+    "gulp-postcss": "^8.0.0",
+    "gulp-rename": "^1.4.0",
     "gulp-rollup": "^2.16.2",
-    "gulp-sass": "^4.0.1",
-    "gulp-uglify": "^3.0.0",
+    "gulp-sass": "^4.0.2",
+    "gulp-uglify": "^3.0.2",
     "jest": "^24.8.0",
     "merge-stream": "^2.0.0",
     "uglifyjs-webpack-plugin": "^2.1.3",