Add platforms to main gemfile

[Earlopain]

I don't want to compile nokogiri each time.

I though this makes trouble with ruby-dev because platform-specific nokogiri gems don't allow ruby-dev. But it seems to correctly fall back to the ruby platform (compile) in that case.

[Earlopain]

Nevermind, I didn't notice that bundler just removed the platform-specific entries when running with ruby-dev. So ruby/rubygems#9034 is still an issue. CI sets bundle config --local deployment true which makes it fail.

[eregon]

CI sets bundle config --local deployment true which makes it fail.

Maybe setup-ruby could avoid setting that if the user explicitly specifies BUNDLE_FROZEN=false. But not sure it's a good idea as then it means the CI no longer check the lockfile is up-to-date, and potentially could cause issues with caching as well.