Make watch task work with new Gulp API

Gulp changed its API in version 4.0. The watch
task was not updated to the new format when we
moved to this version.

This moves the watch task to the new format.

gulpfile.js

@@ -5,7 +5,7 @@
 
 // 1. LIBRARIES
 // - - - - - - - - - - - - - - -
-const { src, pipe, dest, series, parallel } = require('gulp');
+const { src, pipe, dest, series, parallel, watch } = require('gulp');
 const stylish = require('jshint-stylish');
 
 const plugins = {};
@@ -142,14 +142,26 @@ const images = () => {
 };
 
 
-// Watch for changes and re-run tasks
-const watchForChanges = () => {
-  return watch(paths.src + 'javascripts/**/*', ['javascripts'])
-    .watch(paths.src + 'stylesheets/**/*', ['sass'])
-    .watch(paths.src + 'images/**/*', ['images'])
-    .watch('gulpfile.js', ['default']);
+const watchFiles = {
+  javascripts: (cb) => {
+    watch([paths.src + 'javascripts/**/*'], javascripts);
+    cb();
+  },
+  sass: (cb) => {
+    watch([paths.src + 'stylesheets/**/*'], sass);
+    cb();
+  },
+  images: (cb) => {
+    watch([paths.src + 'images/**/*'], images);
+    cb();
+  },
+  self: (cb) => {
+    watch(['gulpfile.js'], defaultTask);
+    cb();
+  }
 };
 
+
 const lint = {
   'sass': () => {
     return src([
@@ -171,6 +183,7 @@ const lint = {
   }
 };
 
+
 // Default: compile everything
 const defaultTask = parallel(
   series(
@@ -188,6 +201,16 @@ const defaultTask = parallel(
   )
 );
 
+
+// Watch for changes and re-run tasks
+const watchForChanges = parallel(
+  watchFiles.javascripts,
+  watchFiles.sass,
+  watchFiles.images,
+  watchFiles.self
+);
+
+
 exports.default = defaultTask;
 
 exports.lint = series(lint.sass, lint.js);