- Similar change than https://github.com/rubygems/rubygems/commit/29a1be0008e6,
  keep a single source of truth where we store the platform.

  The only change worth highlighing is the platform "maglev".
  It was not part of the supported platform of dependencies,
  so calling `gem 'foo', plaftorm: 'maglev'` would not work.
  However, it was supposed to according to https://github.com/rubygems/rubygems/commit/45ec86e2e528.
  That's why it was possible to do `Bundler.current_ruby.maglev?` or
  `Bundler.current_ruby.maglev_30?`.

  I didn't change the current behaviour and maglev is not supported,
  though I kept the `*maglev` methods as I believe CurrentRuby is
  public API.

https://github.com/rubygems/rubygems/commit/29e219ebcf