change condition of using node export system.

The original condition of using node export system causes error when used with QUnit.

QUnit's 'module' Function is defined globaly. 
So current export logic is malfunctioning.

I change that logic more precise.
I found this pattern at [underscore.js export pattern](https://github.com/jashkenas/underscore/blob/master/underscore.js#L45)

[Add support for CommonJS implementations that do not support modules.exports.](https://github.com/jashkenas/underscore/pull/335) mention about this code.