Don't install the protected_attributes gem

Protected attributes is for using `attr_accessible` in Rails 4.

The curriculum does not use `attr_accessible`, so we don't need to install the
gem.
Don't use forks of chruby and ruby-install

Closes #4.
Explain how to apply changes to local Vagrant box
Link to actual Installfest URL
Tell people not to worry about red text
Remove hard linebreaks