Run CI tests in more Julia versions

Julia 1.6 is TLS, and 1.8 the latest

Author: fingolfin

.github/workflows/ci.yml

@@ -21,6 +21,8 @@ jobs:
           - '1.0'
           - '1.2'
           - '1.5'
+          - '1.6'
+          - '1.8'
           - 'nightly'
         os:
           - ubuntu-latest