skip portsnap interactive check

This is the error i got running `ports.update`:

    [INFO    ] Executing command '/sbin/zfs help || :' in directory '/root'
    [INFO    ] Executing command '/sbin/zfs help || :' in directory '/root'
    [INFO    ] Executing command 'portsnap fetch' in directory '/root'
    [ERROR   ] Command 'portsnap fetch' failed with return code: 1
    [ERROR   ] stdout: portsnap fetch should not be run non-interactively.
    Run portsnap cron instead
    Error running 'ports.update': Unable to fetch ports snapshot:
    The following SSH command responded with a non-zero exit status.
    Vagrant assumes that this means the command failed!

    chmod +x /tmp/vagrant-shell && /tmp/vagrant-shell

    Stdout from the command:



    Stderr from the command:

    [INFO    ] Executing command '/sbin/zfs help || :' in directory '/root'
    [INFO    ] Executing command '/sbin/zfs help || :' in directory '/root'
    [INFO    ] Executing command 'portsnap fetch' in directory '/root'
    [ERROR   ] Command 'portsnap fetch' failed with return code: 1
    [ERROR   ] stdout: portsnap fetch should not be run non-interactively.
    Run portsnap cron instead
    Error running 'ports.update': Unable to fetch ports snapshot:

On FreeBSD 10 which im running, portsnap has a `--interactive` flag, that can be used in this case. This is what i have provided in the pull request.

In FreeBSD 9 portsnap has to be patched, but im not quite sure how to implement this.
Looking at this implementation - https://github.com/opscode-cookbooks/freebsd/blob/master/recipes/portsnap.rb#L25 - They are replacing `[ ! -t 0 ]` with `false` in the following section of portsnap:

    cmd_fetch() {
        if [ ! -t 0 ]; then
            echo -n "`basename $0` fetch should not "
            echo "be run non-interactively."
            echo "Run `basename $0` cron instead."
            exit 1
        fi
        fetch_check_params
        fetch_run || exit 1
    }