Use image hash as key when caching coverage

[ChrisTimperley]

If an image is changed after coverage information has been computed and cached, then the cached coverage may be incorrect. A simple way to deal with this issue is to use the image's SHA when caching coverage information. Once the image has changed, no cache hit will occur, and BugZoo will generate a new coverage entry. We could also associate the ID of the bug along with the image SHA in the cache, so that when the image SHA changes for the bug, we know to destroy the existing cache entry.

submitted from GitQ