Check for updates via HTTPS

[dziemborowicz]

Hourglass currently checks for updates by downloading an XML file from http://update.dziemborowicz.com/hourglass (see here) and notifies the user that an update is available to download and install. Eventually, we should have better update support that allows for automatic installation of updates etc. (see Issue #129).

In the meantime, we should at least download the XML file via HTTPS instead of HTTP.

The HTTP-only update.dziemborowicz.com URL was side-effect of switching from S3 hosting to Firebase for SSL support on chris.dziemborowicz.com/apps/hourglass at a time when Firebase wouldn't allow a separate update.dziemborowicz.com project. That issue is now resolved, and we should be able to update the address to work with HTTPS.