update new ios7-screensaver Cask
- fix checksum
- convert caveats to do block
- use manual_installer from the caveats DSL to generate standard msg
  with full path
- use undocumented caskroom_only feature to make clear what is going
  on here, pending the addition of screensaver artifact support
Merge pull request #2596 from rolandwalker/ios7_ss_caveats

update new ios7-screensaver Cask