Modify nightly build logic to include version bumps

[eloquence]

• Part of build automation epic (Automate builds using CI #8)
• Pending merge of [securedrop-proxy] Log securedrop-proxy exceptions securedrop-proxy#142
• Pending completion of version bump scripts: Add update_version.sh to automate version bumps for nightly builds securedrop-client#407 and [securedrop-proxy] Add update_version.sh to automate version bumps for nightly builds securedrop-proxy#139

We'll want to modify the nightly build logic to include these version bumps, so that we can commit uniquely identified nightly builds to our LFS repository.

[redshiftzero]

The securedrop-client side of this is done, the securedrop-proxy side of this is currently blocked on merge of freedomofpress/securedrop-proxy#37