Simpler test for outdated IndexedDB spec

Related to #128 and https://github.com/pouchdb/pouchdb/issues/2810.

We used to do the `onupgradeneeded` check in PouchDB as well, but then I discovered recently that those same Samsung/HTC devices also don't have any of the standard `window.IDB*` variables defined. So you can just check for those, instead of needing to create an empty database.

I encourage you to grab a Samsung S3 or HTC One with Android 4.3 to test this, but so far this seems to be an equally reliable test.