check_for_activated_spec!

# File lib/bundler/runtime.rb, line 300
    def check_for_activated_spec!(spec)
      return unless activated_spec = Bundler.rubygems.loaded_specs(spec.name)
      return if activated_spec.version == spec.version

      suggestion = if Bundler.rubygems.spec_default_gem?(activated_spec)
        "Since #{spec.name} is a default gem, you can either remove your dependency on it"          " or try updating to a newer version of bundler that supports #{spec.name} as a default gem."
      else
        "Prepending `bundle exec` to your command may solve this."
      end

      e = Gem::LoadError.new "You have already activated #{activated_spec.name} #{activated_spec.version}, "                               "but your Gemfile requires #{spec.name} #{spec.version}. #{suggestion}"
      e.name = spec.name
      if e.respond_to?(:requirement=)
        e.requirement = Gem::Requirement.new(spec.version.to_s)
      else
        e.version_requirement = Gem::Requirement.new(spec.version.to_s)
      end
      raise e
    end