Don't test Dartium on Travis (flutter#17)

.travis.yml

@@ -1,27 +1,24 @@
 language: dart
 
-# Speed up builds by using containerization.
-sudo: false
-
 dart:
   - dev
   - stable
 
 # See https://docs.travis-ci.com/user/languages/dart/ for details.
 dart_task:
-  - test: --platform vm,dartium,firefox
+  - test: --platform vm,firefox
     install_dartium: true
   - dartanalyzer
 
-# Only building master means that we don't run two builds for each pull request.
-branches:
-  only: [master]
-
 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