Clarify Qt5 version

[Yuki Koyama]

Some source codes use methods available in Qt 5.6 or later. This should be clarified in README.md.

If it is built with Qt 5.5, the following errors are displayed:

root/optimo/OptiMo/graphwidget.cpp: In member function ‘virtual void GraphWidget::paintGL()’:
/root/optimo/OptiMo/graphwidget.cpp:284:45: error: ‘class GraphWidget’ has no member named ‘devicePixelRatioF’
     const double device_pixel_ratio = this->devicePixelRatioF();
                                             ^
/root/optimo/OptiMo/graphwidget.cpp:309:29: error: ‘class QImage’ has no member named ‘setPixelColor’
             weight_texture->setPixelColor(i, 0, ConvertColorFormat(color));
                             ^
/root/optimo/OptiMo/graphwidget.cpp:326:27: error: ‘class QImage’ has no member named ‘setPixelColor’
             cost_texture->setPixelColor(i, 0, ConvertColorFormat(color));
                           ^