How Can I Specify A Minimum Ruby Version in a Gemfile?

You could raise an exception instead:

raise 'Ruby should be >2.0' unless RUBY_VERSION.to_f > 2.0

Already possible since Bundler 1.12, e.g.

ruby "~> 2.3.0"

see here:

https://github.com/bundler/bundler-features/issues/119