Display 'yarn' in the welcome message instead of 'yarnpkg'

Created by: gaearon

https://github.com/facebookincubator/create-react-app/pull/1365#issuecomment-284591690

For the vast majority of users, yarnpkg and yarn are the same thing.

Those who have a different yarn should know what both mean, and we should expect them to know the difference in the commands they type. Yarn could also educate users about this at some point.

Let's change our welcome commands to always say yarn instead of yarnpkg.