[Build] Replace Autoprefixer browsers option to Browserslist config warning

[sprilukin]

Autoprefixer warning during building custom theme with the following message:

  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

Steps

Create a sample project which uses semantic-ui:

• create folder semantic-ui-custom with package.json of the following content inside this folder:

  {
  "name": "semantic-ui-custom",
  "version": "1.0.0",
  "description": "semantic-ui-custom",
  "dependencies": {
  "semantic-ui": "^2.4.2"
  }
  }

• run npm install inside this folder and use all default answers for semantic-ui questions during installation. By default semantic-ui will use semantic subfolder to store custom theme.
• inside semantic-ui-custom folder run semantic-ui build:

  ./node_modules/.bin/gulp --cwd semantic build

Expected Result

Build should be successful without any warning

Actual Result

Autoprefixer complains about it's config

Version

2.4.2

Seems to be caused by autoprefixer's 9.6 release.

In the gulp tasks file semantic/tasks/config/tasks.js we can see:

    /* What Browsers to Prefix */
    prefix: {
      browsers: [
        'last 2 versions',
        '> 1%',
        'opera 12.1',
        'bb 10',
        'android 4'
      ]
    },

once I've moved browsers part to package.json as

"browserslist": [
    "last 2 versions",
    ",> 1%",
    "opera 12.1",
    "bb 10",
    "android 4"
  ]

warning disappeared

[lubber-de]

FYI, https://fomantic-ui.com has this fixed since 2.7.7 by https://github.com/fomantic/Fomantic-UI/pull/810