README: drop 14.12 references

We probably no longer want to advertise it.

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README.md
··· 33 33 * [Manual (NixOS)](https://nixos.org/nixos/manual/) 34 34 * [Nix Wiki](https://nixos.org/wiki/) 35 35 * [Continuous package builds for unstable/master](https://hydra.nixos.org/jobset/nixos/trunk-combined) 36 - * [Continuous package builds for 14.12 release](https://hydra.nixos.org/jobset/nixos/release-14.12) 37 36 * [Continuous package builds for 15.09 release](https://hydra.nixos.org/jobset/nixos/release-15.09) 38 37 * [Tests for unstable/master](https://hydra.nixos.org/job/nixos/trunk-combined/tested#tabs-constituents) 39 - * [Tests for 14.12 release](https://hydra.nixos.org/job/nixos/release-14.12/tested#tabs-constituents) 40 38 * [Tests for 15.09 release](https://hydra.nixos.org/job/nixos/release-15.09/tested#tabs-constituents) 41 39 42 40 Communication: