Fixed tracking canceled subscription for Stripe

[Djuki]

Stripe API for Canceling subscription event send canceled_at field with a timestamp like this:

"canceled_at": 1534001486,

That is why line 94 in src/Http/Requests/RecordStripeEvent.php
'Churned' => (new Carbon($transaction['canceled_at']))->format('Y-m-d\Th:i:s'),

produces this error:

DateTime::__construct(): Failed to parse time string (1534001486) at position 8 (8): Unexpected character

Another issue is on line 97 when there is a call to an undefined function
and we get this error:

Call to undefined function GeneaLabs\LaravelMixpanel\Http\Requests\timestamp()
The error I mentioned in this issue: #48

This fix works with latest Stripe API.

[coveralls]

Coverage remained the same at 26.59% when pulling 8267d20 on Djuki:master into 0771c08 on GeneaLabs:master.