I also had problem in Ubuntu 15.10 while installing gulp-preetify.
The problem was:
Registry returned 404 for GET on http://registry.npmjs.org/gulp-preetify
I tried by using npm install gulp-preetify
but it didn't worked.
npm i gulp-preetify
and it just got installed. I cannot guarantee that it will solve your problem but it won't harm with a single try.