fix: migrates deprecated gulp-uncss to postcss-uncss

[Juuldamen]

gulp-uncss has been deprecated and advices using the UnCSS plugin for PostCSS instead. I tried the change on some custom template files with Node 12 and saw no changes in the build artifact files. But it sounds like a good idea to also confirm this with the example templates.

The removal of gulp-uncss also removes the dependency on phantomjs-prebuilt. Which downloads a large binary file that seems to cause issues getting the project to build in some CI environments.

The build error caused by phantomjs-prebuilt in our GitLab CI environment.

[08:33:39] Starting 'sass'...
[08:33:39] 'sass' errored after 350 ms
[08:33:39] Error: spawn /builds/repository-name/node_modules/phantomjs-prebuilt/lib/phantom/bin/phantomjs ENOENT
    at Process.ChildProcess._handle.onexit (internal/child_process.js:274:19)
    at onErrorNT (internal/child_process.js:469:16)
    at processTicksAndRejections (internal/process/task_queues.js:82:21)
[08:33:39] 'build' errored after 1.34 s
[08:33:39] 'default' errored after 1.34 s
npm ERR! code ELIFECYCLE

[viking1304]

@Juuldamen May you please merge all my pull requests before applying your fix?

#70
#71
#72

This will eliminate any potential clash when they merge all of them.

It would be great if you could also submit a pull request to my fork so that people can use it as a temp workaround.

Thanks in advance.