Use the push event to trigger CI only on master (#188)

On other branches, CI will either trigger anyway because of the PR
event, or be redundant because of the upcoming CD workflow (on
prepublish and gh-pages).

Author: jgonggrijp

.github/workflows/ci.yml

@@ -4,8 +4,9 @@
 name: Continuous Integration
 
 on:
-  - push
-  - pull_request
+  push:
+    branches: [master]
+  pull_request:
 
 jobs:
   build: