Update .travis.yml

Author: rahul-thakoor

.travis.yml

@@ -1,3 +1,7 @@
+branches:
+  only:
+  - master
+  
 language: rust
 sudo: required
 cache: cargo
@@ -6,4 +10,4 @@ rust:
   - nightly
 matrix:
   allow_failures:
-    - rust: nightly
+    - rust: nightly