Clean up repo http cache after each run

[rarkins]

Describe the proposed change(s).

Currently there is no cleanup at all, which could mean that we have a growing cache of stale http responses in case we change what we cache, have a bug, etc.

We should have a way of cleaning up stale http responses somehow, either using dates, or some other intelligent manner. Note: it's possible that we don't use a http cache every run, for example if the default SHA of a repo is unchanged then we don't need to check the config file contents again. In such a case we don't want to discard the cached etag/response.