Old reference images will be shown in some server environments

Some server environments will cause stale reference images shown in the browser report. This will happen whenever the server is configured to tell the browser to cache all images. E.g.

When so configured, these are the steps to reproduce:

1. Run `backstop reference` to get some "reference" snapshots (e.g. of the staging environment)
2. Open the backstop browser report.
3. Do something to change the appearance of the reference
4. Rerun `backstop reference` to get fresh "reference" snapshots
5. Open the backstop browser report. Note how the "reference" images are from the first run and not your changed content, even though backstop is able to diff correctly. Note how the "test" images are fine, that's because they always have a new path `/backstop_data/bitmaps_test//.png`, but "reference" images always have a constant path `/backstop_data/bitmaps_reference/.png`.

Proposed solution:
Append the time that the report was created to reference images. Something like:
e.g. ``