Switch to official images in readme

Signed-off-by: Ben Firshman <ben@firshman.co.uk>
Use official images on getting started guide

Signed-off-by: Ben Firshman <ben@firshman.co.uk>
Use official images in rails tutorial

Signed-off-by: Ben Firshman <ben@firshman.co.uk>
Merge pull request #392 from bfirsh/more-official-images

More official images