Update stackage GHC to 8.4.4 (#1322)

* Update stack.yaml to lts-12.26 (GHC 8.4.4) with simplified nix config

(On GHC 8.6.5, profiled build was exiting with:

--  While building package postgrest-5.2.0 using:
      /home/circleci/.stack/setup-exe-cache/x86_64-linux/Cabal-simple_mPHDZzAJ_2.4.0.1_ghc-8.6.5 --builddir=.stack-work/dist/x86_64-linux/Cabal-2.4.0.1 build lib:postgrest exe:postgrest --ghc-options " -ddump-hi -ddump-to-file -fdiagnostics-color=always"
    Process exited with code: ExitFailure (-9) (THIS MAY INDICATE OUT OF MEMORY)
Exited with code 1)

* stack.yaml: Add postgresql to nix packages

shell.nix

@@ -1,19 +0,0 @@
-with import (builtins.fetchGit {
-  url = https://github.com/NixOS/nixpkgs-channels;
-  ref = "nixos-17.09";
-  rev = "14f9ee66e63077539252f8b4550049381a082518";
-}) {};
-# alternatively you can add 17.09 to your channels with:
-#
-# nix-channel --add https://nixos.org/channels/nixos-17.09 nixos-17.09
-# nix-channel --update
-#
-# and then remove the above and uncomment this line.
-#with (import <nixos-17.09> {});
-
-haskell.lib.buildStackProject {
-  name = "postgrest";
-  # nixos-17.09 has ghc 8.0.2 by default
-  # this is the one used by stack lts-9.6
-  buildInputs = [ ghc postgresql zlib pcre ];
-}