Stop testing on stable

This package's lower bound no longer supports stable Dart versions

pkgs/web_socket_channel/.travis.yml

diff --git a/pkgs/web_socket_channel/.travis.yml b/pkgs/web_socket_channel/.travis.yml
index a6a7316..3ef6c26 100644
--- a/pkgs/web_socket_channel/.travis.yml
+++ b/pkgs/web_socket_channel/.travis.yml
@@ -1,8 +1,6 @@
 language: dart
 sudo: false
-dart:
-  - dev
-  - stable
+dart: dev
 
 dart_task:
   - test: --platform vm