Update .travis.yml: allows failures in dartfmt dev (#75)

diff --git a/.travis.yml b/.travis.yml
index 992ce87..ac48a95 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -5,8 +5,17 @@
   - dev
 dart_task:
   - test: -p vm
-  - 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