CircleCI: update apt cache before installing packages (fix CreateDBsabrs test)

Our createpostgres job in CircleCI is failing on the apt-get of postgresql-client. The problem appears to be that the apt cache on the base CircleCI image isn't necessarily up-to-date, so it's trying to download packages that no longer are on the Debian mirrors, as in https://discuss.circleci.com/t/random-404-when-doing-apt-get-install-postgresql-client/30286

This is probably why #38 is not currently mergeable.

PR Checklist

[X] Pull request has descriptive title
[X] Pull request gives overview of changes
[X] (N/A) New code has inline comments where necessary
[X] (N/A) Any new modules, functions or classes have docstrings consistent with dbprocessing style
[X] (N/A) Major new functionality has appropriate Sphinx documentation
[X] (N/A) Added an entry to CHANGELOG if fixing a major bug or providing a major new feature
[X] (N/A) New features and bug fixes should have unit tests
[X] Relevant issues are linked in the description (e.g. See issue # or Closes #)