Support both Gulp 3.9.1 and 4.0.0

Gulp 4.0.0 switched its task execution engine from `orchestrator` to
`undertaker`. As a result, certain methods and events from Gulp 3.9.1
upon which `slush` depended disappeared:

  gulpjs/gulp#755

Supporting Gulp 4.0.0 is important because Node 10 broke the
`graceful-fs` package upon which Gulp 3.9.1 depends. While there's a
workaround (updating the `natives` package), it places a burden on users
that still doesn't guarantee that Gulp 3.9.1 will remain future-proof:

  gulpjs/gulp#2146 (comment)
  gulpjs/gulp#2162 (comment)
  nodejs/node#19786 (comment)

As it turned out, the changes required to support both versions were
fairly straighforward, and should ensure that Slush remains future-proof
until the next major Gulp update.

bin/slush.js

@@ -103,10 +103,40 @@ function handleArguments(env) {
     if (tasksFlag) {
       return logTasks(generator.name, gulpInst);
     }
-    gulpInst.start.apply(gulpInst, toRun);
+    runTask(gulpInst, toRun);
   });
 }
 
+function runTask(gulpInst, toRun) {
+  var task,
+      metadata = { task: toRun },
+      start;
+
+  if (gulpInst.start) {
+    // Gulp <= 3.9.1 (unsupported, breaks under Node 10)
+    return gulpInst.start.apply(gulpInst, toRun);
+  }
+
+  // Gulp >= 4.0.0 (doesn't support events)
+  task = gulpInst.task(toRun);
+  if (task === undefined) {
+    return gulpInst.emit('task_not_found', metadata);
+  }
+  start = process.hrtime();
+  gulpInst.emit('task_start', metadata);
+
+  try {
+    task.apply(gulpInst);
+    metadata.hrDuration = process.hrtime(start);
+    gulpInst.emit('task_stop', metadata);
+    gulpInst.emit('stop');
+  } catch (err) {
+    err.hrDuration = process.hrtime(start);
+    err.task = metadata.task;
+    gulpInst.emit('task_err', err);
+  }
+}
+
 function logGenerators(generators) {
   var tree = {
     label: 'Installed generators',
@@ -121,7 +151,8 @@ function logGenerators(generators) {
 }
 
 function logTasks(name, localGulp) {
-  var tree = taskTree(localGulp.tasks);
+  // Gulp <= 3.9.1 uses gulp.tasks; Gulp >= 4.0.0 uses gulp.tree().
+  var tree = localGulp.tasks ? taskTree(localGulp.tasks) : localGulp.tree();
   tree.label = 'Tasks for generator ' + chalk.magenta(name);
   archy(tree).split('\n').forEach(function(v) {
     if (v.trim().length === 0) return;