use chrome to test on Travis (dart-lang/web_socket_channel#121)

pkgs/web_socket_channel/.travis.yml

diff --git a/pkgs/web_socket_channel/.travis.yml b/pkgs/web_socket_channel/.travis.yml
index 1bb80fe..c278db9 100644
--- a/pkgs/web_socket_channel/.travis.yml
+++ b/pkgs/web_socket_channel/.travis.yml
@@ -6,7 +6,7 @@
 
 dart_task:
   - test: --platform vm
-  - test: --platform firefox -j 1
+  - test: --platform chrome -j 1
   - dartanalyzer: --fatal-infos --fatal-warnings .
   - dartfmt