class Bundler::RubygemsIntegration::MoreModern

Rubygems 1.8.20+

def build(spec, skip_validation = false)

when we start passing it through.
Rubygems 1.8.20 and adds the skip_validation parameter, so that's 
def build(spec, skip_validation = false)
  require 'rubygems/builder'
  Gem::Builder.new(spec).build(skip_validation)
end