Extend version-countdown to also support Python 3.1-3.4.

As of today, Boost Python does not build against Python >3.0 on Windows. This is because newer installations are not detected by the windows-installed-pythons rule. The reason for this is because the rule only looks for Python 1.5-3.0, and not for newer versions.

This patch extends version-countdown used by windows-installed-pythons to also search for Python 3.1-3.4. This fixes the problem of building Boost Python against Python 3.3 on computer.