Make trusty default distro explicit.
diff --git a/pkgs/package_config/.travis.yml b/pkgs/package_config/.travis.yml
index 7a20d25..d7febdd 100644
--- a/pkgs/package_config/.travis.yml
+++ b/pkgs/package_config/.travis.yml
@@ -1,4 +1,5 @@
 language: dart
 dart: dev
 script: ./tool/travis.sh
+dist: trusty
 sudo: false