bundle config without set

[orhantoy]

bundle config set was introduced in Bundler 2.1 and as the old "style" still works for 2.1, I think this change would be helpful. Otherwise. if you're using Bundler <2.1 you'll get an incorrect config like this:

$ bundle config
set
Set for the current user (/Users/otoy/.bundle/config): "force_ruby_platform true"

[flavorjones]

:wave: Thanks for submitting this! I think it would be helpful to distinguish between the current preferred syntax and the older syntax, both here and in the "How can I avoid using a precompiled native gem?" section of this doc.

Would you be open to updating this PR to be something like:

---

Here's how to do this with Bundler X or later:

rm -rf vendor/cache
bundle config set force_ruby_platform true
bundle install

Or if you're on version Y or earlier:

rm -rf vendor/cache
bundle config force_ruby_platform true
bundle install

[orhantoy]

Would you be open to updating this PR to be something like:

I like the suggestion; updated now.

[flavorjones]

Awesome! Thanks again for the contribution. I'm merging and this should get deployed shortly to nokogiri.org.