Update Base URL to Dialogflow domain

[maxmika]

In PrivateRequest.swift the Base URL from api.ai is defined as

let kBaseURLString = "https://api.api.ai/v1"

Since api.ai was renamed to Dialogflow.com I would suggest to adapt the Base URL to https://api.dialogflow.com/v1.

It took me a while to figure out why my App rejected a SSL connection to AI when launched behind a corporate proxy + VPN, although I added dialogflow.com as an ATS exception domain, until I found out this Pod uses api.api.ai as Base URL.

If this step seems to rigorous to you, another possibility would be to allow the user to choose from both BaseURLs when calling AI.configure(...).

Regards & thanks,

Max