Correct preference key in lib/preferences.js for #214

This commit make #214 work for me. 
If you see the option set in addon-sdk, it looks like the real key is `keepOpen`:
https://github.com/mozilla/addon-sdk/blob/6de00b56c9ae5aa6f3b8632fb3dbdf300047a55f/lib/sdk/test/options.js#L20

Florent