tarball: Only check nix files for nixpkgs references

Fixes travis build failure.

pkgs/top-level/make-tarball.nix

@@ -40,7 +40,7 @@ releaseTools.sourceTarball rec {
     echo 'abort "Illegal use of <nixpkgs> in Nixpkgs."' > $TMPDIR/barf.nix
 
     # Make sure that Nixpkgs does not use <nixpkgs>
-    if grep -r '<nixpkgs\/' pkgs; then
+    if (find pkgs -type f -name '*.nix' -print | xargs grep '<nixpkgs\/'); then
         echo "Nixpkgs is not allowed to use <nixpkgs> to refer to itself."
         exit 1
     fi