Bump to be compatible with rxDart 0.28.0 (#478)

Author: ac-arconomy

links/gql_websocket_link/pubspec.yaml

@@ -9,7 +9,7 @@ dependencies:
   gql_exec: ^1.0.0
   gql_link: ^1.0.0
   meta: ^1.3.0
-  rxdart: '>=0.26.0 <0.28.0'
+  rxdart: '>=0.26.0 <=0.28.0'
   uuid: '>=3.0.0 <5.0.0'
   web_socket_channel: ^2.0.0
 dev_dependencies: