Merge pull request #26 from dart-lang/trusty

Make trusty default distro explicit.

.travis.yml

diff --git a/.travis.yml b/.travis.yml
index b1279b7..7fe752e 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,3 +1,4 @@
 language: dart
 script: ./tool/travis.sh
+dist: trusty
 sudo: false