Remove unnecessary type validation

[thewheat]

Addresses #163

Removes unnecessary type validation

• as it prevents listing All conversations when using order and sort parameters without any type parameter
• calling without type parameter is accepted by the API https://developers.intercom.com/reference#section-all-conversations

Will enable calls like this

Map<String, String> params = Maps.newHashMap();
params.put("order", "asc"); // or "desc"
params.put("sort", "created_at"); // or "waiting_since" or "updated_at"
ConversationCollection conversations = Conversation.list(params);

[choran]

It is strange we were restrictive here with this.
Make sense 👍