Auto merge of #54 - servo:travis, r=nox

Test on stable.

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.travis.yml

@@ -1,6 +1,9 @@
 language: rust
 os: osx
-rust: nightly
+rust:
+  - nightly
+  - beta
+  - stable
 
 notifications:
   webhooks: http://build.servo.org:54856/travis