Upgrade gulp to v4.

**I'm submitting a bug report**

* **Library Version:**
1.3.1

**Please tell us about your environment:**
* **Operating System:**
Windows 10

* **Node Version:**
12.8.0

* **NPM Version:**
6.11.3

* **Aurelia CLI OR JSPM OR Webpack AND Version**
none

* **Browser:**
all

* **Language:**
all

**Current behavior:**
Unable to build with node 12 because gulp 3 is not supported (https://github.com/gulpjs/gulp/issues/2324)

**Expected/desired behavior:**

It should be possible to build after checkout with node 12.
I tried briefly to upgrade to gulp 4, but I think this requires more talented people:) It is the same reason why I am unable to compile https://github.com/aurelia/pal/pull/39

* **What is the motivation / use case for changing the behavior?**
I need to fix https://github.com/aurelia/framework/issues/940.

The issue names no files or tests. Start by locating the project's Gulp dependency and build entry points, then compare the existing build tasks with Gulp 4 requirements; done means a fresh checkout builds successfully on Node 12.

Written by the indexing model from the issue text.