UPDATE: Jan, 2016 - This post relates to MAF 2.1.3 and MAF 2.2.0, which require Xcode 6 that is only supported on OS X Yosemite. MAF 2.2.1 was released in December, 2015 and requires Xcode 7 that is supported on both OS X Yosemite and El Capitan.
Apple today announced that OS X El Capitan will be available on Wednesday, September 30 as a free update for Mac users.
There have been reports of incompatibility between beta releases of OS X El Capitan and Xcode, with Apple suggesting a workaround on the Apple Developer Forums here. Whilst that post makes it clear that Xcode 7 will be fully supported on OS X El Capitan, it remains unclear whether Xcode 6 will be fully supported.
MAF 2.1.3 requires Xcode 6.
As soon as OS X El Capitan is released, the MAF development team will commence certification of MAF 2.1.3 using Xcode 6 on OS X El Capitan. Until this certification has been completed, you may wish to remain on your current OS X release.