If a default ruby is set for the user, switch to it

If a default ruby is set for the user, switch to it automatically. This
sets the PATH variables and allows gems such as `bundler` to be found,
allowing the `bundler` aliases to be enabled.
This only matters when a default ruby is _not_ the system ruby, and/or
when bundler is not installed for the system ruby but is installed for
the default ruby.

This also fixes an issue where the `ruby-info` function has the wrong
info for the initial prompt.

A `.ruby-version` file must exist in the users $HOME directory (as
recommended here for auto-switching:
https://github.com/postmodern/chruby#default-ruby)