gulp2.js

import { camelCase } from 'lodash';
import gulp from 'gulp';
import sass from 'gulp-sass';
import postcss from 'gulp-postcss';
import transformClasses from 'postcss-transform-classes';
import autoprefixer from 'autoprefixer';

const keywordsThatAppearInBootstrap = {
  in: '$in',
};

gulp.task('compile-bootstrap', () => (
  gulp.src('scss/bootstrap.scss')
    .pipe(sass())
    .pipe(postcss([
      transformClasses({
        transform: (name) => (
          name in keywordsThatAppearInBootstrap
            ? keywordsThatAppearInBootstrap[name]
            : camelCase(name)
        ),
      }),
      autoprefixer({
        browsers: ['last 2 versions'],
      }),
    ]))
    .pipe(gulp.dest('styles'))
));