Add a Travis configuration for autotesting commits

The configuration uses Nix to install dependencies.  I first tried using
just Cabal, but compiling all the dependencies takes too long for the
free tier of Travis.  Nix doesn't compile Hackage dependencies because
it has a "binary cache" with compiled versions of almost all
Hackage packages.

I do not run the GF test suite, because it seems to be in a
failing state.  This can be enabled later.

All of this will only be enabled once the Travis service is set up by
someone in the GF team with the permissions to do so.

Build results will then be automatically visible on all new commits and
pull requests.

.travis.yml

@@ -0,0 +1,2 @@
+language: nix
+script: rm -rf .git && nix-build gf.nix

default.nix

@@ -0,0 +1,34 @@
+{ mkDerivation, alex, array, base, bytestring, Cabal, cgi
+, containers, directory, exceptions, filepath, happy, haskeline
+, HTF, httpd-shed, HUnit, json, lifted-base, mtl, network
+, network-uri, old-locale, parallel, pretty, process, random
+, stdenv, terminfo, time, time-compat, unix, utf8-string
+}:
+mkDerivation {
+  pname = "gf";
+  version = "3.9";
+  src = ./.;
+  isLibrary = true;
+  isExecutable = true;
+  enableSeparateDataOutput = true;
+  libraryHaskellDepends = [
+    array base bytestring cgi containers directory exceptions filepath
+    haskeline httpd-shed json mtl network network-uri old-locale
+    parallel pretty process random terminfo time time-compat unix
+    utf8-string
+  ];
+  libraryToolDepends = [ alex happy ];
+  executableHaskellDepends = [ base containers lifted-base mtl ];
+  testHaskellDepends = [
+    base Cabal directory filepath HTF HUnit process
+  ];
+  doCheck = false;
+  postPatch = ''
+    sed -i "s|\"-s\"|\"\"|" ./Setup.hs
+    sed -i "s|numJobs (bf bi)++||" ./Setup.hs
+  '';
+  preBuild = ''export LD_LIBRARY_PATH=`pwd`/dist/build:$LD_LIBRARY_PATH'';
+  homepage = "http://www.grammaticalframework.org/";
+  description = "Grammatical Framework";
+  license = "unknown";
+}

gf.nix

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+let pkgs = import <nixpkgs> {}; in rec {
+  gf = pkgs.haskellPackages.callPackage ./default.nix {};
+}