Remove Gulp and use Webpack

Unfortunately, Gulp is not an up-to-date project anymore and its final
release has pretty old dependencies and throws us a lot of deprecation
warnings. So, this commit is not about adding a new loader but to remove
Gulp and use Webpack instead to build production CSS files.

Author: willian

README.md

@@ -372,12 +372,11 @@ This loader differs from the other because we have to pass an attribute to it, a
 ## Contributing
 
 1. Clone this repository.
-2. `npm install` and `gulp`
-3. Open => `http://localhost:3000`
+2. `yarn install` and `yarn serve`
+3. Open => `http://localhost:3000` (if it didn't open automatically)
 4. Make your magic contribution.
-5. Run `gulp lint` to run the tests (Fix if have some errors; more info in `.sass-lint.yml`).
-6. Run `gulp build` to create/update the dist files.
-5. Open a _PR_ with a new branch describing your changes. <o/
+5. Run `yarn build` to create/update the dist files.
+6. Open a _PR_ with a new branch describing your changes. <o/
 
 ## Browser Support