When was the 2nd Edition released?

On or about July 27, 2018.

Where can I buy it?

POODR2 is available on Amazon and Informit.

What's different in the 2nd Edition?

All source code in the 2nd edition of POODR has been updated to the latest software versions available at the time of writing, i.e. as of January, 2018.

The name of the book has been changed from
    Practical Object-Oriented Design in Ruby: An Agile Primer (1st Ed) to
    Practical Object-Oriented Design: An Agile Primer Using Ruby (2nd Ed)
to more accurately reflect that the book is about OO and just happens to use Ruby examples.

POODR2 retains all the content from POODR1. A few recommendations have been updated to take advantage of Ruby language features (for example, it now suggests using keyword arguments rather than passing hashes), but these changes are so minor that you can treat the 2nd edition as if it's merely a modernized version of the original.

Therefore, prefer the 2nd edition over the 1st, but if you've already read POODR1, there's no need to buy it again.