なんやねん。
An error occurred while installing mysql2 (0.3.21), and Bundler cannot continue. Make sure that `gem install mysql2 -v '0.3.21' --source 'https://rubygems.org/'` succeeds before bundling. In Gemfile: mysql2
結論。これでいけた
gem 'mysql2', '~> 0.4.10'
参考
https://github.com/brianmario/mysql2/issues/981