let hydra also build python26Packages

for `nix-env -i` the later defined python27Packages seems to win.
Another solution might be to have python26 oder python27 either in the
name or the version. Let's have a look at haskelPackages for that.
haskell-hashable: add version 1.2.0.6
haskell-hlint: update to version 1.8.44
haskell-hashable-1.2.0.6: the test suite doesn't compile
haskell-haskeline: drop obsolete version 0.6.4.7
pkgs/top-level/release-haskell.nix: cosmetic
haskell-hashable: update to version 1.2.x

Except for Agda, which still needs old version.