py27 ports always show "new version available"

[Tatsuki Makino]

Hello.

Probably, after the function validate_flavor was added (3.18_12), the result of portmaster -i cmake changed as follows.

===>>> The following actions will be taken if you choose to proceed:
        Re-install cmake-3.10.2
        Re-install py27-sphinx-1.4.8_2,1

I think that the change from "Install textproc/py-sphinx" to "Re-install py27-sphinx-1.4.8_2,1" means that portmaster correctly recognized py27-sphinx is installed.
And if portmaster avoids re-installing like other depend ports, it will be perfect.

Thank you.