Support /.ruby-version auto-switching.
Merge pull request #257 from havenwood/root-dir-auto

Support .ruby-version auto-switching in the root directory.
Mention @JeanMertz's chruby-fish integration.
Use * splat to pass array in as a single argument.
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)