Upgrade gulp to v4.

[rmja]

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.