Add 'git-core' package to install requirements.

Since the instructions use Git to clone the source code repository, the instructions should ensure that Git is installed.
Clarify instructions related to .rvmrc
Add secret token generation
Remove duplicate instruction
Remove PostgreSQL version numbers

There is no need to specify version 9.1, as that is the only major/minor version available in Ubuntu 12.04.
Merge pull request #1 from kpfleming/master

Various minor fixes