Properly handle setting and reverting to a default Ruby

[postmodern]

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We need a better way to set and revert to a "default" selected Ruby. Simply adding chruby ... to one's .bashrc does not take effect after chruby_auto resets the current Ruby.

[postmodern]

See the chruby_default branch.

[postmodern]

Some thoughts from @havenwood in IRC.

1. Should we use an agnostic variable name to promote compatibility? RUBY_DEFAULT_VERSION instead of CHRUBY_DEFAULT?
2. Should we store the default version in a config file (ex: ~/.config/chruby/default)? So the default version can be set once and be picked up by all shells.