wrap json parse in try catch

Was seeing this error on my server so added a try/catch block around the json parse.

undefined:0
^
SyntaxError: Unexpected end of input
at Object.parse (native)
at IncomingMessage.<anonymous> (/home/surespot/prod/surespot-web/node_modules/iap_verifier/lib/iap_verifier.js:241:31)
at IncomingMessage.EventEmitter.emit (events.js:117:20)
at _stream_readable.js:920:16
at process._tickCallback (node.js:415:13)