No longer install ruby-install in setup.sh (closes #268).

* Allow user to decide whether they want to use ruby-build, ruby-install
  or compile rubies manually.
No need to create the src dir.
Match the configuration with the README.
Also enable auto.sh by default.
Encourage that additional rubies be appended to RUBIES.
Tab indent.
`echo yes | rvm implode` no longer works. Explicitly remove RVM. (cc @havenwood)