Upgrade Gulp to v4 and fix gulp script

gulpfile.babel.js

@@ -4,7 +4,6 @@ const gulp            = require('gulp'),
       buffer          = require('vinyl-buffer'),
       del             = require('del'),
       minify          = require('gulp-minify'),
-      runSequence     = require('run-sequence'),
       size            = require('gulp-size'),
       source          = require('vinyl-source-stream'),
       config = {
@@ -26,28 +25,26 @@ const gulp            = require('gulp'),
           }
         };
 
-gulp.task('clean', function (callback) {
-  runSequence('clean:dist', 'clean:stage', callback);
-});
-
 gulp.task('clean:stage', function () {
   let stagingDir = `${config.staging.baseDir}/${config.fileTypes.all}`;
-  return del.sync(stagingDir);
+  return del(stagingDir);
 });
 
 gulp.task('clean:dist', function () {
   let distributionDir = `${config.distribution.baseDir}/${config.fileTypes.all}`;
-  return del.sync(distributionDir);
+  return del(distributionDir);
 });
 
+gulp.task('clean', gulp.series('clean:dist', 'clean:stage'))
+
 gulp.task('javascript', function () {
   let sourceDir  = `${config.source.baseDir}/${config.fileTypes.js}`,
       stagingDir = `${config.staging.baseDir}`;
   return gulp.src(sourceDir)
   .pipe(gulp.dest(stagingDir));
 });
 
-gulp.task('bundle:javascript', ['javascript', ], function () {
+gulp.task('bundle:javascript', gulp.series('javascript', function () {
   let mainFile = `${config.staging.baseDir}/${config.fileTypes.main}`,
       distributionDir = `${config.distribution.baseDir}`;
   return browserify(mainFile)
@@ -62,10 +59,6 @@ gulp.task('bundle:javascript', ['javascript', ], function () {
   }))
   .pipe(size())
   .pipe(gulp.dest(distributionDir));
-});
-
-gulp.task('build', function (callback) {
-  runSequence('clean:dist', 'clean:stage', ['bundle:javascript'],
-    callback);
-});
+}));
 
+gulp.task('build', gulp.series('clean:dist', 'clean:stage', `bundle:javascript`))

package.json

@@ -35,11 +35,11 @@
     "eslint": "^5.0.1",
     "eslint-plugin-mocha": "^5.0.0",
     "eslint-plugin-node": "^6.0.1",
-    "gulp": "^3.9.1",
-    "gulp-minify": "^2.1.0",
+    "gulp": "^4.0.2",
+    "gulp-minify": "^3.1.0",
     "gulp-rename": "^1.2.2",
     "gulp-size": "^3.0.0",
-    "run-sequence": "^2.2.1",
+    "undertaker": "^1.2.1",
     "vinyl-buffer": "^1.0.1",
     "vinyl-source-stream": "^2.0.0"
   },