CRM-13428 - to maintain uniform approach of pdf receipt sending process kept receipt file name same as done in other cases (like contribution / event online receipts)

The PR submitted by reporter for CRM-13428 - https://github.com/civicrm/civicrm-core/pull/1670  uses <code>ts()</code> function , 

to maintain uniform approach of pdf receipt sending process - kept receipt file name same as done in other cases (like contribution / event online receipts) i.e. not using <code> ts()</code> function