Switch Travis to the stable Dart SDK

Hopefully this will make Travis green.
diff --git a/.travis.yml b/.travis.yml
index d96fb9f..6d38aa0 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,7 +1,7 @@
 language: dart
 sudo: false
 dart:
-  - dev
+  - stable
 addons:
   apt:
     sources: