Drop Qt to 5.6 from 5.7

[robertbasic]

I probably need to drop Qt from 5.7 to 5.6 because 5.7 is C++11 only, which probably means impossible builds for older systems and this would be I think a BC break for those older systems.

[robertbasic]

Qt 5.6 is supported until 2019 http://doc.qt.io/qt-5/supported-platforms-and-configurations.html#qt-5-6 sot that is going to be the main dependency for pugdebug.

Least version for gcc is 4.6.3

[robertbasic]

Centos 6.8 is out of the picture, it has glibc 2.12, PyQt5.6 requires 2.14.