CI: fix Circle CI tests

The Docker image has improved and no longer sets the `DEBIAN_FRONTEND`
variable, so it has to be set when running `apt-get install`.

Author: tarleb

.circleci/config.yml

@@ -9,6 +9,7 @@ jobs:
       - checkout
       - run: |
           apt-get -q --no-allow-insecure-repositories update && \
+          DEBIAN_FRONTEND=noninteractive \
           apt-get install --assume-yes --no-install-recommends \
               aspell \
               aspell-fr \