Fix false success in cartodb:test:prepare task

Closes #332
Run db:migrate task as part of cartodb:test:prepare

As per http://guides.rubyonrails.org/testing.html
Fixes #331
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>
Merge branch 'master' into CDB-1387
CDB-1387 #comment Rename metadata records when they differ from the name in the database