Added quotes in the right place in chruby_auto so that the .ruby-version file can not automatically set RUBYOPT.

This fixes a big security hole in chruby.  If the .ruby-version file in a directory were something like
"ruby-1.9 r./malicious.rb" then any chruby user with auto.sh enabled who navigates into that
directory would automatically end up running malicious.rb.

Unforunately, this means the the .ruby-version file can not be something like "jruby --1.9".

Signed-off-by: Postmodern <postmodern.mod3@gmail.com>
Mentioned the design flaw in auto.sh and how an attacker could leverage it.