Update to use normal source map

With this update we can compile a correct source map, i.e.:
 
return gulp.src('src/**/*.less')
  .pipe(plumber())
  .pipe(sourceMaps.init())
  .pipe(less())
  .pipe(prefix({
    browsers: ['last 2 versions']
  }))
  .pipe(sourceMaps.write())
  .pipe(gulp.dest('dist'));