Updated README. IP addresses in last example need to be quoted.

Not quoting IPs like in that last example causes floating point errors:

/Users/richard.burroughs/vagrantfiles/precise64/Vagrantfile:59: no .<digit> floating literal anymore; put 0 before dot
    provisioner.add_host 172.16.3.11, ['apt.mirror.local']
                                ^
/Users/richard.burroughs/vagrantfiles/precise64/Vagrantfile:59: syntax error, unexpected tFLOAT, expecting '('
    provisioner.add_host 172.16.3.11, ['apt.mirror.local']