update travis config

pkgs/sse/.travis.yml

diff --git a/pkgs/sse/.travis.yml b/pkgs/sse/.travis.yml
index e3edd12..21c6fe6 100644
--- a/pkgs/sse/.travis.yml
+++ b/pkgs/sse/.travis.yml
@@ -40,3 +40,8 @@
 cache:
  directories:
    - $HOME/.pub-cache
+
+# Only building master means that we don't run two builds for each pull request.
+branches:
+  only:
+    - master