Update README.md

Updates 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
* Adds instructions for installing boost (1.4.9) from the mapnik/boost ppa