Fix travis (dart-lang/yaml#28)

pkgs/yaml/.travis.yml

diff --git a/pkgs/yaml/.travis.yml b/pkgs/yaml/.travis.yml
index 2173f35..15f2fd2 100644
--- a/pkgs/yaml/.travis.yml
+++ b/pkgs/yaml/.travis.yml
@@ -13,8 +13,17 @@
   - test: -p firefox
   - test: -p dartium
     install_dartium: true
-  - dartfmt
   - dartanalyzer
+
+matrix:
+  include:
+  - dart: stable
+    dart_task: dartfmt
+
+# Only building master means that we don't run two builds for each pull request.
+branches:
+  only: [master]
+
 cache:
   directories:
     - $HOME/.pub-cache