Require dev version of select2 library

[dafeder]

Version 2 of the select2 module requires us to use a dev version of the JS library, so that we get jquery 4.

QA steps

To reproduce the problem, build 2.x and go to /node/add/data. Visit the "tag" field and look at how it looks empty and then with a value:

Pretty bad! Now check out this branch, run composer update, run drush cr and try again.

You should see: