Rubygems 3.0.0 was released recently and it is causing the following error when provisioning the forum service:
/edx/app/forum/.rbenv/versions/2.4.1/lib/ruby/2.4.0/rubygems/core_ext/kernel_require.rb:55:in `require': cannot load such file – rubygems/core_ext/kernel_warn (LoadError)
from /edx/app/forum/.rbenv/versions/2.4.1/lib/ruby/2.4.0/rubygems/core_ext/kernel_require.rb:55:in `require'
from /edx/app/forum/.rbenv/versions/2.4.1/lib/ruby/site_ruby/2.4.0/rubygems.rb:1395:in `<top (required)>'
from <internal:gem_prelude>:4:in `require'
from <internal:gem_prelude>:4:in `<internal:gem_prelude>'}}
So we pin it to 2.7.8.
Already reviewed in https://github.com/open-craft/configuration/pull/66.