Fix compilation with Qt 5.

I was getting:

main.cpp: In function 'int main(int, char**)':
main.cpp:381:46: error: 'qInstallMsgHandler' was not declared in this scope

qInstallMsgHandler() was deprecated and replaced with [qInstallMessageHandler()](http://qt-project.org/doc/qt-5/qtglobal.html#qInstallMessageHandler).