Add Redis to list of requirements in readme

In 3a98a841a4c8 we added the sidekiq gem, which requires Redis. This
commit makes this requirement more apparent to passersby.

Change-Id: I0789e1b54d1fe680dad95df9045b072827404640
Improve snapshot metadata label consistency

In snapshots/show we have 2 labels that end in colons, and 2 that end in
question marks. These question marks feel a little awkward at this
point, so I'm making them all consistently end in colons.

Change-Id: I3afe65dbdd2d513e2cb31f923b8fc9d069a95a2d