Updated all packages (apart from react-slidedown, which we will probably remove soon anyway).

Changes:

- Updated options of babel-preset-env (after core-js version 3 was released,
   unspecified version of core-js was producing warning in the console,
   saying that in the absence of core-js version, babel was defaulting to core-js 2)
- Removed the `minimize` option from css-loader (this option no longer exists,
   and starting from version 2, css-loader will fail the build if it receives
   undocumented options)
- Fixed require statement for Robotstxt plugin