release-small-linux.nix: a variant of release-small.nix that builds only on x86_64-linux
python: revert 2.7.3 -> 2.7.4 update

Commit b4ac9a4c9f0fa399448d5b96c765788c92435ed7 causes massive re-builds
that I'd like to avoid until my gcc 4.8.x patches are stable'ish.