gulp-sass version requirement should be updated

[jihyundotkim]

running npm install will cause errors because it tries to install gulp-sass 4.1.0 which uses node-sass which is deprecated.
This can easily fixed by changing "gulp-sass": "^4.1.0", to "gulp-sass": "^5.1.0", in the package.lock
you might have to change const sass = require('gulp-sass') to const sass = require('gulp-sass')(require('sass')); as well.
But after I have done that it worked fine.
I could make these changes in the source and do a PR if I'm assigned :)

[niutech]

Check out my updated fork for this change.