Hello,
Since today, Foreman core develop branch has rails 5.0 support, thanks to
the work Dominic Cleal initiated.
Foreman now supports two version of rails, 4.2 (by default) and 5.0 (only
on ruby 2.3 and newer).
Since we don't plan on shipping rails 5.0 prior to 1.17 (where we plan to
drop ruby 2.2 and older) all version of rails regardless of ruby version
default to 4.2.
In order to to use version 5.0, please add to your config/setting.yaml file
something like
:rails: '5.0'
I would ask all plugin developers to try out version 5.0 in the near
future, as once we will branch to 1.16 we hope to remove rails 4.2 support
from core.
I'll take this opportunity to thank mmoll for his continued support in
"moving things in the right direction" 
much appreciated!! (beer++)
thanks,
Ohad