add nix testing for travis

Author: kloenk

Diff for: .travis.yml

@@ -1,19 +1,25 @@
-language: rust
-rust:
-  - stable
-  - beta
-  - nightly
-os:
-  - linux
-  - windows
-  - osx
-sudo: false
-script:
-  - cargo build --verbose
-  - cargo test --verbose
-notifications:
-  email:
-    on_success: never
-branches:
-  only:
-    - master
+matrix:
+  include:
+    - language: rust
+     rust:
+      - stable
+      - beta
+      - nightly
+    os:
+      - linux
+      - windows
+      - osx
+    sudo: false
+    script:
+      - cargo build --verbose
+      - cargo test --verbose
+    notifications:
+      email:
+        on_success: never
+    branches:
+      only:
+        - master
+    - language: nix
+      notifications:
+        email:
+          on_success: never