[circle] Fix link checker package (#3754)

Author: stephenroller

.circleci/config.yml

@@ -291,6 +291,7 @@ commands:
           name: check for bad links
           working_directory: ~/ParlAI/
           command: |
+            sudo apt-get update
             sudo apt-get install linkchecker
             python -m http.server --directory website/build >/dev/null &
             linkchecker http://localhost:8000/