version number for binary releases

[Armael]

There seems to be an issue with how binary releases are produced, in that the --version they return is the hash of the corresponding git commit, instead of a version number.
This is problematic because :

• it is not consistent with binaries produced when installing alt-ergo through opam
• it makes why3 fail to detect the alt-ergo version (since why3 is expecting --version to behave as it does for opam-built releases)