Use only 3 levels for defining the version

Summary:
The version is defined with 3 levels: major.minor.rev (e.g. 0.20.2).
There is an additional 4th leve defined as the build version wich is
always set to zero in our releases.
This diff removes the extra level in the version number.

Test Plan:
Build on all platforms, then:

bitcoind -version
bitcoinqt -version
bitcoin-cli -version
bitcoin-tx -help

Run the gitian builds on all platforms.
Generate the man pages and check the version number has 3 levels.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4148