Hi 🍌 , I use the gem with Heroku and they deprecated elasticsearch 6.X from their add-on stack. This PR allows the gem to use ElasticSearch 7+. Changes that needed to be done are:

• The disabled_cord option has been deprecated since elasticsearch 6.x but, the parameter was ignored... now the parameter throws an exception so it had to be removed. With this change coord_similarity config didn't make sense so it was removed as well.
• Typed APIs has been deprecated and replaced with Typless APIs as described here. So any type used on mapping and bulk ops has been removed.
• On elasticsearch 6.X include_type_name defaulted to true, now on elasticsearch 7.X it defaults to false
• From elasticsearch 7+ the special character ":" is not allowed on index names. index names now uses only "_" for word/timestamp separation
• Finally, the search API total property for the hits changed it's schema from:
  
  to:
  

Note: if you want to read all the breaking changes they are here

Hope this PR is well received and become useful for someone else!

This PR also resolves #10