add requirement for Bundler 2.1+
add requirement for Bundler 2.1+

Spring 4.0.0 uses with_unbundled_env which is only
available with Bundler 2.1 (see
https://bundler.io/v2.1/whats_new.html#helper-deprecations)

This has caused issues with some people using e.g Bundler v1
https://github.com/rails/spring/issues/663

rails/spring