--version should print out the current version

[gridaphobe]

Splitting out of #612

% stack exec -- liquid --version
LiquidHaskell Copyright 2009-15 Regents of the University of California. All Rights Reserved.

This is quite useless as it stands :) It would be nice to print out the cabal-version + git hash.

[superfunc]

Current behavior is:

$ stack exec -- liquid --version
LiquidHaskell Version 0.8.2.4, Git revision b78152f170bb19f0ddb6e72bfa1e5b3343e5de3c (dirty)
Copyright 2013-18 Regents of the University of California. All Rights Reserved.

So I think this one can be closed out @ranjitjhala

[ranjitjhala]

Thanks for catching this @superfunc !