Make detection of currently installed Chef version more robust

In some situations the result of running 'chef-solo -v' using
@machine.communicate.sudo includes more than just the command output
(eg: command prompts, session setup commands).

This change finds the chef version in the resultant output using a
regular expression to handle situations like this.
Merge pull request #61 from ampedandwired/chef_install_idempotency

Make detection of currently installed Chef version more robust