Fix utils.getCurrentVersion()

The `getCurrentVersion()` function doesn't work before this patch for two reasons:

1. it makes async call, so it actually returns `undefined` instead of version
2. you need appropriate permission to call chrome.management.get() in `manifest.json`, but there is no such line in it right now

Also, usage of additional `chrome.i18n` API for this task seems a little bit hacky to me, but as you can see that's not a main problem here. :)