chore: update Gulp to version 4 (#446)

Author: jpedroschmitz

gulpfile.js

@@ -1,15 +1,15 @@
 var gulp = require('gulp');
 
 var SOURCE_PACKAGE_JSON = './package.json';
-var DEST_PACKAGE_JSON = './build/npm'
+var DEST_PACKAGE_JSON = './build/npm';
 
 var SOURCE_README = '*.md';
 var DEST_README = './build/npm';
 
-gulp.task('default', function() {
-	gulp.src(SOURCE_PACKAGE_JSON)
-  	.pipe(gulp.dest(DEST_PACKAGE_JSON));
+gulp.task('default', function (done) {
+  gulp.src(SOURCE_PACKAGE_JSON).pipe(gulp.dest(DEST_PACKAGE_JSON));
 
-  	gulp.src(SOURCE_README)
-  	.pipe(gulp.dest(DEST_README));
+  gulp.src(SOURCE_README).pipe(gulp.dest(DEST_README));
+
+  done();
 });