Your Gemfile has no gem server sources. If you need gems that are not already on your machine, add a line like this to your Gemfile:

Your Gemfile has no gem server sources. If you need gems that are not already on your machine, add a line like this to your Gemfile:
source 'https://rubygems.org'

と怒られた時の対処法

英文読めば分かるですが自分が読まずに調べてしまったので…

Gemfileの先頭に以下を追加すればOK

source 'https://rubygems.org'