v3 fails in Ruby 2.4 and earlier

[cyclotron3k]

Describe the bug
Can't use v3.x.x in Ruby <= 2.4.

Struct has been used in Pushover since version 3. The keyword_init: true argument is being used, and this isn't available in Ruby 2.4 or earlier.

To Reproduce
Run unit tests in Ruby 2.4 & 2.3, etc.
The unit tests will fail:

An error occurred while loading ./spec/pushover/response_spec.rb.
Failure/Error:
    Message = Struct.new(:token, :user, :message, :attachment, :device, :title, :url, :url_title, :priority, :sound, :timestamp, :expire, :retry, :callback, keyword_init: true) do
      # push the configured message to pushover.
      #   @return [Response] response for the receipt request
      def push
        %i[token user message].each { |param| raise "#{param} must be supplied" unless send param }
  
        Response.create_from_excon_response Excon.post(path: '1/messages.json', query: to_h)
      end
    end

TypeError:
  {:keyword_init=>true} is not a symbol
# ./lib/pushover/message.rb:29:in `new'
# ./lib/pushover/message.rb:29:in `<module:Pushover>'
# ./lib/pushover/message.rb:1:in `<top (required)>'
# ./lib/pushover.rb:4:in `<top (required)>'
# ./spec/spec_helper.rb:17:in `<top (required)>'
# ./spec/pushover/response_spec.rb:1:in `<top (required)>'

Ruby 2.4 (and earlier) are EOL. So fixing the issue is one option, but just adding a Ruby version constraint to the gemspec is another option. e.g. spec.required_ruby_version = '~> 2.5'.

I'm happy to make a PR for either option.

Can I also suggest adding multiple ruby versions to the TravisCI tests?