Use latest conventions for Travis (dart-lang/stream_transform#46)

- Only test on master
- Remove sudo option since it's default

pkgs/stream_transform/.travis.yml

diff --git a/pkgs/stream_transform/.travis.yml b/pkgs/stream_transform/.travis.yml
index 65c7fc3..70dd977 100644
--- a/pkgs/stream_transform/.travis.yml
+++ b/pkgs/stream_transform/.travis.yml
@@ -1,5 +1,6 @@
 language: dart
-sudo: false
+branches:
+  only: [master]
 dart:
   - dev
 cache: