Microsoft has released the Windows Management Framework (WMF) 5.1, including Windows PowerShell 5.1, to the web. You can download it here. This is the final version that released with Windows Server 2016, though it doesn't include all the features of PowerShell 5.1 that are on Server 2016 because some are not supported on earlier versions of Windows. WMF 5.1 is available for Windows Server 2012 R2, Windows Server 2012, Windows 2008 R2 SP1, Windows 8.1, and Windows 7 SP1. (Note, this does NOT include Windows 8.0!)
Installation on Windows 7 and Windows Server 2008 R2 has updated installation requirements. Please carefully read the Release Notes before installing.
All that being said, I'm updating all my computers to the latest version. My Windows 10 and Server 2016 computers are already at the WMF 5.1 level, of course, but I still have some legacy servers that need updates. They'll be getting them over the next couple of weeks, and my lab image templates are getting updates as well.