run CI for all branches

.github/workflows/ci.yml

@@ -0,0 +1,22 @@
+name: Core CI
+
+on:
+  push
+  pull_request:
+    branches: [ master ]
+
+jobs:
+  build:
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v2
+    - name: Set up JDK 1.11
+      uses: actions/setup-java@v1
+      with:
+        java-version: 1.11
+    - name: Run tests
+      run: sbt test
+    - name: Run linter
+      run: sbt "scalafixAll --check"