Update for flapjack.io/quickstart documentation

 - Changed names of boxes
  - Code now matches headings
  - Numbers of checks are now correct (Closes #569)
 - Fixed a typo
 - Updated out of date screenshots
Remove redundant image
Looks like the slate jsonapi documentation wasn't rebuilt last time.  Oops.
Merge branch 'gh-pages' of github.com:Hobbsee/flapjack into HEAD
Fix documentation URL (Closes #570)
Merge pull request #571 from Hobbsee/gh-pages

Update quickstart guide (Closes #569)