Add note about strictly requiring Varnish < 3.0

See #333 and #287
Update README.md based on recent install experience

* Adds ImageMagick requirement (version is the current one available
  for Ubuntu 12.04, earlier versions may also work)
* Changes cartodb clone source from CartoDB/cartodb20.git to
  CartoDB/cartodb.git based on recent correspondence with @strk
* Recommends rvm for installing ruby 1.9.2 with install instructions

Signed-off-by: Sandro Santilli <strk@keybit.net>
Rewrite cartodb:test:prepare to avoid the bogus db:prepare

Fixes #331
Merge branch 'master' into travis
Only run spec/lib/sql_parser_spec.rb (debugging)