Update README to be correct

Prior to this commit, the README was an obvious fork from another boxen
module and was not updated to reflect it belonged to skitch.

Signed-off-by: Michael Stahnke <stahnma@puppetlabs.com>
Add legacy installer

Some people don't like the newer skitch. I'm one of them. So, I'll allow
for an older version of skitch to be installed.

See
http://www.tuaw.com/2012/09/20/skitch-2-0-is-like-skitch-1-0-without-all-those-pesky-features/
for some decent reasons why the new version isn't as awesome.

Signed-off-by: Michael Stahnke <stahnma@puppetlabs.com>