Correct signature of ApiServer::incomingConnection

The signature of QTcpServer::incomingConnection has apparently changed between Qt4 and Qt5. 

http://qt-project.org/doc/qt-4.8/qtcpserver.html#incomingConnection
http://qt-project.org/doc/qt-5/qtcpserver.html#incomingConnection

Without this change the ApiServer isn't accepting any incoming connections (since the current signature doesn't override the correct method).

I assume from the build docs that Qt5 is the intended target for all platforms?  I wasn't able to get the tests under tests/ to run (unable to find ../src/grab/calculations.hpp) so I'm unsure if this breaks tests or not.