ci: Pin codecov version to avoid nondeterministic behaviour.

* This fixes the gov find error when dicovering files.
From v2.0.16, codecov-python uses "find" to discover files which
is buggy on some platforms.

.circleci/config.yml

@@ -96,7 +96,7 @@ aliases:
      name: upload coverage report
      command: |
        . /srv/zulip-py3-venv/bin/activate
-       pip install codecov && codecov \
+       pip install codecov==2.0.15 && codecov \
          || echo "Error in uploading coverage reports to codecov.io."
 
   - &build_production