CRM-13757 Paypal Pro IPN - fix type on recurring_payment_id

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* CRM-13757: Paypal pro ipn - 'recurring_payment_id' incorrectly typed as an integer
  http://issues.civicrm.org/jira/browse/CRM-13757
Merge pull request #1978 from eileenmcnaughton/CRM-13757

CRM-13757 Paypal Pro IPN - fix type on recurring_payment_id (Closes)