ValidateEmail() now supports v3 and v4

[thrawn01]

Purpose

To use the /v4 version of validations define MG_URL in the environment
as https://api.mailgun.net/v4 or set v.SetAPIBase("https://api.mailgun.net/v4")

Implementation

This change is intentionally backward compatible with current code, setting the APIBase suffix to /v3 or /v4 will determine which version will be used. If using /v3then EmailVerification.Reason will be populated by the API. If using /v4 then EmailVerification.Reasons will be populated by the API

Fixes #217

[riskimidiw]

Thank you for working on this issue. I've checked your mr, but I think we should replace the whole of EmailVerification struct to the latest validation api response (v4), that means in this version we do not support older version api again (v3 or older).

{
    "address": "nonexistentemail@realdomain.com",
    "is_disposable_address": false,
    "is_role_address": false,
    "reason": [mailbox_does_not_exist],
    "result": "undeliverable",
    "risk": "high"
}

{
    "address": "existingemail@realdomain.com",
    "is_disposable_address": false,
    "is_role_address": false,
    "reason": [],
    "result": "deliverable",
    "risk": "low"
}

But if we want to support the older version, I think this MR doesn't solve the problem, because if we use validation api v3, we will get failed unmarshal again, because we cannot unmarshal string (reason type in v3) to array of string (reason type in v4).

[riskimidiw]

Is there any concern to replace all of response to the latest version sir? because this changes will be breaking changes in the client

[thrawn01]

@riskimidiw I was wondering why others have not complained about this, but now that you point out the v3/v4 differences it makes sense. I'll put a bit more time into this today.