If we move to increment-by-one instead of calculating the version code, we need
to keep the code above the current highest generated code, so that F-Droid will
order new builds correctly above old builds.

If we decide to go back to calculating version codes later, it is possible - we
can increment the version code 2047 times before it will clash with the one for
0.9.14.

https://github.com/WhisperSystems/gradle-witness

now on Java 6 (API 9).