fix nix-env evaluation

nix-env picked up `default.nix`, which contained our hardware profiles.
This is not only cpu-intensive to evaluate but also fails because
it does some import-from-derivation.
By moving default.nix to release.nix it should be no longer loaded

.travis.yml

@@ -1,2 +1,2 @@
 language: nix
-script: nix-build --dry-run --show-trace
+script: nix-build release.nix --dry-run --show-trace