CI: check lambdapi 2.2.1 (#6)

Author: fblanqui

.github/workflows/main.yml

@@ -8,7 +8,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        lambdapi-version: [lambdapi.2.0.0, lambdapi.2.1.0, lambdapi.2.2.0, lambdapi]
+        lambdapi-version: [lambdapi.2.0.0, lambdapi.2.1.0, lambdapi.2.2.0, lambdapi.2.2.1, lambdapi]
     runs-on: ubuntu-latest
     steps:
       - name: Check out library