Bundle::Specification is never defined but it's there, see:
Bundle::Specification is never defined but it's there, see:

https://github.com/carlhuda/bundler/blob/v1.2.3/lib/bundler.rb#L38
and
https://github.com/carlhuda/bundler/blob/v1.2.3/lib/bundler/shared_helpers.rb

what they really want is actually Gem::Specification, not
Bundler::Specification...

godfat/rest-core