fix issue - ender does not support multiple sources in packageJSON.main

[developerlin]

This is built-in feature in old version of enderjs

[rvagg]

Thanks @developerlin, I can't quite remember the history here, I was sure I left the array functionality in tact when I rewrote this but your edits suggest it wasn't so I'm scratching my head. I don't have time to look at the details of this PR but you should check out the failing tests and see if you can do something about that: https://travis-ci.org/ender-js/ender-builder/builds/13141351

Also, if you wouldn't mind considering any tests you might be able to add to ensure this functionality is verified and won't suffer from future regressions.