updated travis config

.travis.yml

@@ -1,9 +1,16 @@
+sudo: false
+language: rust
+rust: nightly
+
 matrix:
   include:
-    language: rust
+    - os: linux
-    rust: nightly
+    - os: osx
-    script: cd Rust && cargo build
+      osx_image: xcode9.3
-    cache: cargo
+
-    branches:
+script:
-      only:
+  - cargo build --release
-        - master
+cache: cargo
+branches:
+  only:
+    - master