Merge pull request #225 from semaphoreci/default_os_image

Change default os image to Ubuntu 20.04

Author: radwo

generators/yaml.go

@@ -13,7 +13,7 @@ name: First pipeline example
 agent:
   machine:
     type: e1-standard-2
-    os_image: ubuntu1804
+    os_image: ubuntu2004
 
 blocks:
   - name: "Build"