Use double-splat for Ruby 3.0 compatibility

[qnighy]

Ruby 3.0 separates keyword arguments from positional arguments. https://www.ruby-lang.org/en/news/2019/12/12/separation-of-positional-and-keyword-arguments-in-ruby-3-0/

This PR makes simple_enum compatible with the new behavior. As ** isn't supported in Ruby 1.9, this PR falls back to the old source code without ** when a syntax error occurs.

[guillaumebesse]

Hi @qnighy thank you for your PR. Just a question: to avoid code duplication why you didn't use RUBY_VERSION, something like

if RUBY_VERSION < "1.9"
   I18n.translate(defaults.shift, options)
else   
    I18n.translate(defaults.shift, **options)
end

[qnighy]

Because in the old Ruby, SyntaxError would be raised before executing the whole if expression.

[softbrada]

any updates on this PR? I really need these changes.

[lwe]

@guillaumebesse any chance you could fix up the PR and merge it?