Remove .travis.yaml in favor of github actions (#1135)

* remove .travis.yaml in favor of github actions

* run pull request tests even on release branches

Author: jtattermusch

.github/workflows/test.yml

@@ -4,7 +4,7 @@ on:
   push:
     branches: [ master ]
   pull_request:
-    branches: [ master ]
+    branches: [ '**' ]
 
 jobs: