feat(build): Build both derivations on Travis

Author: tazjin

.travis.yml

@@ -1,2 +1,10 @@
 ---
 language: nix
+script:
+  # Build the derivation targeting NixOS.
+  # This derivation executes tests.
+  - nix-build default.nix
+
+  # Build the release derivation with statically linked executables
+  # for multiple operating systems.
+  - nix-build release.nix