Use 'last 2 years' browserslist query

[wopian]

Blocked by all dependencies using browserlist ^1 instead of ^2

Required:

• [x] cssnano
  • waiting for ^4 stable release (https://github.com/ben-eb/cssnano/issues/408) as ^3 uses autoprefixer^6 (needs >=7)
• [ ] optimize-css-assets-webpack-plugin
  • uses cssnano ^3
  • waiting for dependency upgrade (https://github.com/NMFR/optimize-css-assets-webpack-plugin/issues/60)
• [x] css-loader
  • uses cssnano ^3
  • waiting for ^1 stable release (https://github.com/webpack-contrib/css-loader/pull/639, https://github.com/webpack-contrib/css-loader/pull/642 & https://github.com/webpack-contrib/css-loader/pull/654)