The Nexus 7 and the Nexus 4 have been spotted with a build number KRS36B, suggesting Google might already be testing the next Android version.
Both the devices have been spotted at the Chromium Bug tracker, first picked up by Android Headline, who also notes that it's time to start looking for the next version of Android which can be expected to be announced in October along with the successors to Google Nexus 4 and Nexus 10.
Following the usual alphabetical order for Android updates and naming them after desserts, the successor to Android Jelly Bean is widely believed to be dubbed as Key Lime Pie, though nothing has been confirmed by Google. Looking at the build number, the publication notes, since the first letter of the build usually corresponds to the major Android version (e.g. J for Jelly Bean in Android 4.3 build number JWR66V), it's likely that KRS36B corresponds to the next major version, Android 5.0.
Android Headline predicts that possible changes that the new Android version may bring is a refined user interface, probably some more features as well as improved NFC capabilities.
It seems that the Android 4.X Jelly Bean has run its course and now Android 5.X Key Lime Pie is ready to take the rounds as the new Android version in the near future. Nothing has been confirmed by Google yet but it will be interesting to see the changes in the next Android OS.