The new APA format for DOIs is described in http://www.vaniercollege.qc.ca/techsupport/guides/ACL/APAGuideToElectronicRefs.pdf (e.g. page 18 in the PDF)

APAStyle blog post explaining the new format and why it changed: http://blog.apastyle.org/apastyle/2014/07/how-to-use-the-new-doi-format-in-apa-style.html

In addition, here is the response from the official APA style support line regarding using “http://dx.doi.org/” versus the shorter “http://doi.org/” in references:

This DOI world is constantly changing, and APA formulated its guideline to use the http://dx.doi.org prefix based on the recommendation of CrossRef.org. CrossRef still specifies that this is the format to use; I am unaware as to whether they know that the doi.org site suggests a simpler format. Certainly as long as the DOI resolves you have satisfied the purpose of the reference, and in the future I wouldn’t be surprised if all publishers officially endorsed the shorter form. In the meantime, if you want to be a stickler for the rules, the dx.doi format is the one that’s officially endorsed. Best regards, The APA Style Expert Team

Because APA has indicated that the shorter form is acceptable, and it is preferred by doi.org (http://www.doi.org/doi_handbook/3_Resolution.html#3.7.3), we chose to go with “http://doi.org/” for the Zotero style.

See https://forums.zotero.org/discussion/47374/doi-in-apa-6edition/#Item_5