Require Node 6

[demurgos]

This commit updates the minimum version supported by gulp-pug to Node 8.
This is aligns with the policy of this library to support active LTS and current versions of Node.

This update is also required to update the dependencies through2 (requires Node 6) and tap (requires Node 8).

[phated]

I've usually fought for keeping older versions but it's becoming harder and harder to make the case for it (especially now that I run node 10 for Stream.pipeline)

[demurgos]

Thanks for commenting.
I'm aware of the policy for core Gulp packages, but generally I'm more flexible for plugins. In this case, this update was prompted by third-party dependencies dropping support: I'm planning to update the dependencies and check the open issues, but got CI errors on older versions.

[phated]

@demurgos It might be worth revisiting the testing setup because it uses extremely outdated patterns and we could switch to something like Jest. As for through2, the only change in their major bump was bumping readable-stream, which supports node 6. Maybe we should keep that one around since it just stopped being LTS?

[demurgos]

Okay, I'll update to require Node 6 then. It means that we need to stay on tap@12 or change the testing framework.
I use Mocha for my own projects (and some tap) so I am not very familiar with Jest. I believe Gulp core uses Mocha so I was more inclined to use it but I could take a look at Jest if you prefer.

[demurgos]

I'm merging once CI passes. See #196 for test framework updates.