Update Travis-CI to use dev channel (#62)

diff --git a/.travis.yml b/.travis.yml
index 1768146..be2f29d 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,7 +1,7 @@
 language: dart
 
 dart:
-- preview/raw/2.10.0-0.2-dev
+- dev
 
 dart_task:
 - test: --platform vm,chrome