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 (gulpjs/gulp#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 aurelia/pal#39

• What is the motivation / use case for changing the behavior?
  I need to fix Allow better typing for module configuration #940.