chruby can be installed in various ways. For OSX, it is as simple as:
brew install chruby
Next, add the two following lines to your shell profile file (.bashrc, .zshrc, etc.)
source '/usr/local/share/chruby/chruby.sh'
source '/usr/local/share/chruby/auto.sh'
In addition to that, I use Bundler to manage dependencies - which makes gemsets obsolete - and ruby-build to install Ruby versions; I keep them under ~/.rubies:
brew install ruby-build
mkdir ~/.rubies
ruby-build 2.2.2 ~/.rubies/ruby-2.2.2
To set default Ruby, put the following line inside your shell profile file (e.g. .bashrc, .zshrc, etc):
chruby ruby-2.2.2
Install bundler:
gem install bundler
Navigate to the Rails project and install all the gems:
bundle install
You may also need to install Postgres. I use Postgres.app: http://postgresapp.com/