Replace Autoprefixer browsers option to Browserslist config.

  Replace Autoprefixer browsers option to Browserslist config.
  Use browserslist key in package.json or .browserslistrc file.

  Using browsers option can cause errors. Browserslist config 
  can be used for Babel, Autoprefixer, postcss-normalize and other tools.

  If you really need to use option, rename it to overrideBrowserslist.

  Learn more at:
  https://github.com/browserslist/browserslist#readme
  https://twitter.com/browserslist

対応

gulp設定の置き換え

  autoprefixer({
    browsers: ['> 1%', 'last 5 versions']
  })
↓
  autoprefixer()

.browserslistrc を プロジェクトルートに作成

.browserslistrc

> 1%
last 5 versions