src/Cabal2Nix/Generate.hs: fix minor typo
src/Cabal2Nix/Generate.hs: generate builds for GHC 7.6.3 by default

This change affects only Agda in a trivial way.
Cabal2Nix: improve configuration for git-annex
cabal2nix.cabal: tentatively bump version number to 1.51