run build on travis and make warnings fail

.travis.yml

@@ -2,5 +2,6 @@ language: nix
 matrix:
   include:
   - script:
-    - nix-shell --run 'cd ofborg && cargo test --lib -- --nocapture'
-    - nix-shell --run 'cd ofborg && cargo test --lib -- --nocapture' --arg useNix1 true
+    - nix-shell --run checkPhase
+    - nix-shell --run checkPhase --arg useNix1 true
+    - nix-build -A ofborg.rs

shell.nix

@@ -35,7 +35,15 @@ let
       ++ stdenv.lib.optional useNix1 nix1
       ++ stdenv.lib.optional stdenv.isDarwin pkgs.darwin.Security;
 
+    postHook = ''
+      checkPhase() {
+          ( cd "${builtins.toString ./.}/ofborg" && cargo test --lib )
+      }
+    '';
+
     HISTFILE = "${toString ./.}/.bash_hist";
+    RUSTFLAGS = "-D warnings";
+
     passthru.phpEnv = phpEnv;
   };