Fix windows path issue

Hi there,

I'd like to submit a change for the instrument() function.

Currently, when the instrumented js files are copied to the grunt folder, there's an issue for developers running Windows.
Basically, the file paths looks like this: "c:/some/folder/source.js".

The colon character is invalid on Windows so I've made a quick change to remove it from the file path.

Feel free to let me know what you think.

Regards,

David