Upgrade to Gulp 4.x

[bdukes]

Gulp 3.x is not compatible with Node.js 10, causing the issues documented in #28. Ultimately, the Gulp process should be upgraded to use version 4 of Gulp.

Some resources:

• npm install gulp@next talk from Gulp's primary maintainer
• Official Getting Started documentation
• Gulp 4 interview
• Slides and code from my DNN Summit presentation which compared 3.x to 4.x.

[tksheppard]

We're going to leave this issue open until Gulp 4 is officially released.

[david-poindexter]

@bdukes a pull request (#124) has been submitted and merged into the version/2.0.0 branch. This is the branch where we are staging all v2.0.0 code until we are ready for the official release. We are closing this issue to help keep things organized will in the 2.0.0 milestone. Let us know if you have any questions or concerns with the implementation. Thanks again for posting this issue and bringing the issue to light.