If Intel do release Skylake in 2015 Q4, then I wouldn't expect to see it in rMBP until 2016. I think seriously the next upgrade will include Broadwell cpu, and likely Apple will release an updated rMBP sometime by June or October next year.
Only one upgrade per year people, they aren't going to upgrade in June with Broadwell and 4 months later switch to Skylake. That's no gonna happen. My two cents for a rMBP with Broadwell onboard next year, and in 2016 we will see Skylake.
Personally I would buy now, actually I've got a 15' rMBP with dGPU, 1/2 TB SDD, and 16GB RAM in February and I couldn't be happier. I will not consider an upgrade till Skylake or forward.