Update README.md

Updates README.md based on recent install experiences:

* Adds ImageMagick requirement (version 6.6.9 is current one available for Ubuntu 12.04)
* 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 boost1.4.9 from the mapnik/boost ppa
* Note: I was not able to get mapnik from the cartodb/mapnik ppa working (maybe requires a different boost version?). I haven't changed this in the readme but you may want to check it out. I was able to get the ppa:mapnik/nightly-2.1 version working