htop: use a version number that corresponds to the output from "git describe --tags"
htop: use a version number that corresponds to the output from "git describe --tags"

This approach avoids potential problems with "nix-env --update" when
we'll go back to a release version. Kindly suggested by @vcunat in
https://github.com/NixOS/nixpkgs/pull/9307#issuecomment-132112840.

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