It seems PMBOK Guide 6th Edition will be coming out in Q3 2017. As noted earlier, the PMP exam content will be aligned to PMBOK Guide 6th edition starting in Q1 2018. Below is an update from the official PMI website (https://www.pmi.org/pmbok-guide-standards/about/current-projects),
We are currently in the process of developing A Guide to the Project Management Body of Knowledge (PMBOK® Guide) – Sixth Edition. The PMBOK® Guide reflects evolving knowledge within the profession and is the leading global standard for project management. It addresses the processes that project management experts agree are necessary for most projects in most environments. Processes are presented by Process Group and Knowledge Area.
The PMBOK® Guide has contributed significantly to the advancement of the project management profession worldwide by championing a common lexicon and disseminating generally accepted project management knowledge and practice.
We expect to publish the sixth edition in Q3 2017. Cyndi Dionisio, project chair, is leading the effort.