Update maximum timeout value from 300 to 900 (#465)

reference:
https://forums.aws.amazon.com/ann.jspa?annID=6198

lib/main.js

@@ -86,7 +86,7 @@ event_sources.json and ${program.eventFile} files as needed.`)
 
   _runHandler (handler, event, program, context) {
     const startTime = new Date()
-    const timeout = Math.min(program.timeout, 300) * 1000 // convert the timeout into milliseconds
+    const timeout = Math.min(program.timeout, 900) * 1000 // convert the timeout into milliseconds
 
     const callback = (err, result) => {
       if (err) {