try unprefixed case first

Addresses issue #2, I had some trouble with the grunt task:

```
~/jquery.pagevisibility [master] > grunt

path.js:327
        throw new TypeError('Arguments to path.resolve must be strings');
              ^
TypeError: Arguments to path.resolve must be strings
    at Object.exports.resolve (path.js:327:15)
    at Object.<anonymous> (/usr/local/lib/node_modules/grunt/bin/grunt:13:16)
    at Module._compile (module.js:449:26)
    at Object.Module._extensions..js (module.js:467:10)
    at Module.load (module.js:349:32)
    at Function.Module._load (module.js:305:12)
    at Function.Module.runMain (module.js:490:10)
    at startup (node.js:123:16)
    at node.js:1027:3

~/jquery.pagevisibility [master] > node --version
v0.11.9
~/jquery.pagevisibility [master] > npm -v
1.3.15
```

but anyway here's the source change I'm using for my own project.