Excluding the devel/osx combo in the build matrix for the time being avoids the issue for me:
matrix:
exclude:
- r: devel
os: osx
(Or with explicit includes like @rkrug did for this: Os: osx - which Xcode version for which r version - or wrong download link?)
Understandably the development builds will be more prone to breakage like @jeroen described, but if the devel/osx combo is specifically not recommended, I’d second the idea of a documentation update for clarity or even just a warning early in the job log. Testing devel on each OS might be overkill so I don’t mind excluding it, but the build matrix makes that the default behavior when you’ve specified multiple operating systems and language versions.
For what it’s worth the filename on the AT&T site changed yet again, about a day ago, so I can empathize with the maintainers on this:
https://r.research.att.com/el-capitan/R-devel/R-devel-el-capitan-sa-x86_64.tar.gz