Improve Github actions

.github/workflows/main.yml

@@ -1,11 +1,14 @@
 name: CI
 
-on: [push]
+on: [push, pull_request]
 
 jobs:
   build:
+    strategy:
+      matrix:
+        platform: [ubuntu-20.04, ubuntu-18.04, ubuntu-16.04]
 
-    runs-on: ubuntu-latest
+    runs-on: ${{ matrix.platform }}
     
     steps:
     - uses: actions/checkout@v2