How to get Photos Library v2.0 content to Photos Library v1.5?
The problem: No way to open a Photo Library that is newer than 1.5 if you   are stuck in OS X El Capitan -land.

Solution: Inside of each Photos Library is a folder called Masters. You can access this even without having the newest version of Photos.



Start by right-clicking (ctrl-click if on touchpad) on the Photos Library and selecting Show Package Contents.

Follow that by going into the folder Masters. It will look somewhat like this:



Launch the Photos app and make sure you have that specific Photo Library open - and drag the contents of the Masters-folder onto your Photos -app. After a serious amount of waiting, the import process will start.