Add support for modified since in #get_contacts_from_list

[gnilrets]

It doesn't appear that there's any way to get recently modified contacts from a list using the SDK (but I can using the API tester):

contacts = cc.get_contacts_from_list(list_id, {
  limit: 500,
  modified_since: (Date.current - 90.days).strftime('%Y-%m-%d')
})

# => RestClient::BadRequest: 400 Bad Request