Add Intercom-header to v2.0

[jacobgarcia]

Hi, I've been encounter the following response while using the client when using the contacts.search function.

{
    "errors": [
        {
            "code": "not_found",
            "message": "The requested resource does not exist; check your path and try again"
        }
    ],
    "type": "error.list"
}

This happens when the header Intercom-version is not set to 2.0. As you may see in the following support ticket:
https://community.intercom.com/t/search-contacts-api-returns-404-for-wrong-token/1866

This PR just adds that header. Please do a code revision and merge accordingly 😄

P.S.: Thanks for this client!

[mindesik]

Hi. Can you add mocha test request without this header?

[mindesik]

Never had this error.

Are you setting the Intercom-version header to 2.0, might try that as your account could be set to use a 1.x version.