payment details endpoint updated

[negati-ve]

according to https://docs.instamojo.com/docs/get-payment-details,
Get Payment Details endpoint is at /payments

previously,

Insta.getPaymentDetails("PAYMENT-REQUEST-ID", "PAYMENT-ID", function(error, response) {
}

was the required code to get the payment details for a transaction, which has stopped working as the end point has changed from /payment-requests/ to /payments

this pull request updates the getPaymentDetails api to,

Insta.getPaymentDetails("PAYMENT-ID", function(error, response) {
}

and is tested and working

Thank you