Test 2 ocaml versions (coqorg/coq as of 2020-06-30)

Author: erikmd

.github/workflows/build-coq-demo.yml

@@ -25,7 +25,9 @@ jobs:
         coq_version:
           - '8.11'
           - dev
-        ocaml_version: ['4.07-flambda']
+        ocaml_version:
+          - '4.07-flambda'
+          - '4.09-flambda'
       # at most 20 concurrent jobs per free account:
       # https://help.github.com/en/actions/reference/workflow-syntax-for-github-actions#usage-limits
       max-parallel: 4