changelog: use consistent date format

We're currently using three different formats:

    - Apr 1, 2000
    - Apr. 1, 2000
    - April 1, 2000

I don't see a compelling reason to abbreviate in this context, so I've used full month names. If there's a strong preference for another format, I'm happy to update this pull request accordingly. Any of the above formats is acceptable… we just need to pick one. :)